node.js style REST client function for unity3d. paste bellow into your MonoBehaviour.

Post.cs

protected delegate void OnResponse(string error, string body);
protected void Post(string url, Dictionary<string, string> options, OnResponse onResponse)
{
    StartCoroutine(PostFlow(url, options, onResponse));
}

protected IEnumerator PostFlow(string url, Dictionary<string, string> options, OnResponse onResponse)
{
    if (options == null)
    {
        options = new Dictionary<string, string>();
    }

    var form = new WWWForm();
    foreach (var pair in options)
    {
        form.AddField(pair.Key, pair.Value);
    }
    var www = new WWW(url, form);
    yield return www;

    string body = "";
    if (string.IsNullOrEmpty(www.error))
    {
        body = www.text;
    }

    onResponse(www.error, body);
}

sample.cs

Post("http://hoge", new Dictionary<string, string>{{"key", "value"}, {"key2", "value2"}}, (error, body) => {
    // hogehoge
});