Add Playlist-api XML Schema

[gregsochanik]

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.

var response = new FluentApi<Playlist>()
                .ForPlaylistId(id)
                .ForUser(token, secret)
                                .UsingMethod("DELETE")
                .Response();

As the playlist-api supports accessing the same uri via multiple Http verbs

[gregsochanik]

All tests pass