A GruntFile.js for minifying CSS using Yahoo!'s yuicompressor and minifying JS using Google's Closure compiler. Note that the Javascript gets compiled to a single file while the minified css files remain separate.

gruntFileJsMinifyModule.js

module.exports = function(grunt) {
	grunt.registerTask('compile-js', 'Runs GoogleClosure compiler on all javascript files', function() {
		var exec = require('child_process').exec;
		var done = grunt.task.current.async();
		var compilationLevel = "--compilation_level SIMPLE_OPTIMIZATIONS ";
		var cmd = "java -jar ./build/google-closure-compiler.jar " + compilationLevel;
		var jsDir = "./javascript/";
		var addJs = "--js " + jsDir;
		var output = " --js_output_file ./build/deploy/javascript-compiled.js";
		// Javascript files must be in the same order as loaded in the html file
		var jsFiles = [
			addJs + 'jquery-2.0.2.js' // The first line needs the addJs prefix
            // Add other javascript files to this array
		].join(" " + addJs);
		var run = cmd + jsFiles + output;

		grunt.log.writeln(run);
        
		exec(run, function(err, stdout, stderr) {
			if(stdout) console.log(stdout);
			if(stderr) console.log("ERROR:" + stderr);
			done(err);
		});
	});

	grunt.registerTask('min-css', 'Minifies css using yuicompressor', function(){
		var exec = require('child_process').exec;
		var done = grunt.task.current.async();
		var subCmd = "java -jar ./build/yuicompressor-2.4.7.jar ";
		var output = " -o ./build/deploy/compiled.";
		var minCss = ".min.css";
		var cssDir = "./css/";
		// Exclude the file extension from the name: property
		var cssFiles = [
			{
				dir: cssDir,
				name: "flatui" //Add the names of the CSS files without the .css extentsion
			}
            // Add an object for each css file to this array
		];
		var cmd = "";
        
		cssFiles.forEach(function(val, idx, arr) {
			cmd += subCmd + val.dir + val.name + ".css" + output + val.name + minCss + ";";
		});
        
		grunt.log.writeln(cmd);
        
		exec(cmd, function (error, stdout, stderr) {
			if(stdout) console.log(stdout);
			if(stderr) console.log("ERROR:" + stderr);
			done(error);
		});
	});
};

Forgot to mention you need to download both the YuiCompressor and Google Closure compiler executable jar files before using this GruntFile

Oh and you need Java installed as well.... OpenJDK is fine if you hate Oracle that much lol