T4Template post processor due to Unity does not support T4Template. (copied from http://pastebin.com/vmGQJeLk)

T4TemplatePostProcessor.cs

using System;
using System.Collections;
using System.Collections.Generic;
using System.IO;

namespace UnityEditor.T4Templating
{
	public sealed class T4TemplatePostProcessor : AssetPostprocessor
	{
		[MenuItem("Assets/Force T4 Detection", priority=1000)]
		public static void OnGeneratedCSProjectFiles()
		{
			string[] Starts = { "<Compile Include=\"" , "<None Include=\"" };
			const string End = "\" />";
			string[] GeneratedLines = {
@"<AutoGen>True</AutoGen>",
@"<DesignTime>True</DesignTime>",
@"<DependentUpon>{0}</DependentUpon>",
									  };
			string[] TemplateLines = {
"<None Include=\"{0}\">",
"<Generator>TextTemplatingFileGenerator</Generator>",
"<LastGenOutput>{1}</LastGenOutput>",
"</None>",
									 };
			const string ItemGroupEnd = "  </ItemGroup>";
			List<string> TemplatesFiles = null;
			List<string> TemplateTargets = null;
			bool AnyTemplates = false;
			bool AnyModifications = false;
			string projectDirectory = Directory.GetCurrentDirectory();
			const string searchPattern = "Assembly-CSharp*-vs.csproj";
			const SearchOption searchOption = SearchOption.TopDirectoryOnly;
			string[] projectFiles;
			try
			{
				projectFiles = Directory.GetFiles(projectDirectory, searchPattern, SearchOption.TopDirectoryOnly);
			}
			catch(System.Exception e )
			{
				UnityEngine.Debug.LogException(e);
				UnityEngine.Debug.LogError(projectDirectory + " " + searchPattern + " " + searchOption );
				return;
			}
			foreach (var file in projectFiles)
			{
				string tempProjectFile = FileUtil.GetUniqueTempPathInProject() + " " + Path.GetFileName(file);
				using (var reader = new StreamReader(file))
				using ( var writer = new StreamWriter(tempProjectFile) )
				{
					string line;
					while ( (line = reader.ReadLine()) != null )
					{
						if( line.EndsWith(End) )
						{
							bool ActedOn = false;
							foreach( var Start in Starts )
							{
								int startIndex = line.IndexOf(Start);
								if( startIndex != -1 )
								{
									startIndex += Start.Length;

									int lengthFromIndex = line.Length - ( startIndex + End.Length );
									// there should be no quotes in here..
									if (line.IndexOf('\"', startIndex, lengthFromIndex) == -1)
									{
										string path = line.Substring(startIndex, lengthFromIndex);

										string withTT = Path.Combine(
											Path.GetDirectoryName(path),
											Path.GetFileNameWithoutExtension(path) + ".tt" );

										if (File.Exists(withTT))
										{
											string line_stripped_exec = line.Remove(line.Length - 2, 1);

											int starterPos = Start.IndexOf('<') + 1;
											string manual_exec =
												string.Concat(
													Start.Remove(starterPos),
													"/",
													Start.Substring(starterPos, Start.IndexOf(' ', starterPos) - starterPos),
													">");

											object ttFileName = Path.GetFileName(withTT);

											writer.WriteLine(line_stripped_exec);

											AnyModifications = true;

											foreach (var generatedLineInsert in GeneratedLines)
											{
												writer.WriteLine(generatedLineInsert, ttFileName);
											}
											writer.WriteLine(manual_exec);

											if (!AnyTemplates)
											{
												TemplatesFiles = new List<string>();
												TemplateTargets = new List<string>();
												AnyTemplates = true;
											}
											TemplatesFiles.Add(withTT);
											TemplateTargets.Add(Path.GetFileName(path));
											ActedOn = true;
											break;
										}
									}
								}
							}
							if (ActedOn)
								continue;
						}
						else if (AnyTemplates && line == ItemGroupEnd)
						{
							object[] args = new object[2];

							for (int c = TemplatesFiles.Count, i = 0; i < c; i++)
							{
								args[0] = TemplatesFiles[i];
								args[1] = TemplateTargets[i];

								foreach (var templateLine in TemplateLines)
									writer.WriteLine(templateLine, args);
								AnyModifications = true;
							}
							AnyTemplates = false;
						}
						writer.WriteLine(line);
					}
				}
				if (AnyModifications)
				{
					File.Copy(tempProjectFile, file, true);
				}
				File.Delete(tempProjectFile);
			}
		}
	}
}

Use it for just reference or only on a special case. Unity recommends to create separated visual studio solution for T4Template project not inside a unity project folder.