Add Playlist-api XML Schema  

This adds the the playlist-api schema, which takes advantage of the new multi content-type request payload functionality as well as using new http verbs.

Currently works when specifying the Http verb in the fluent call i.e.

```c#
var response = new FluentApi<Playlist>()
				.ForPlaylistId(id)
				.ForUser(token, secret)
                                .UsingMethod("DELETE")
				.Response();
```