Add more support for playlists

[mschoneman]

Adds three new methods on IPlexServerClient to support retrieving playlist items, creating playlists, and adding items to a playlist. A new sample is included that shows how to use these new methods to split a large playlist into smaller playlists. This is useful to overcome the 24h sync limit in PlexAmp large playlists.

[jensenkd]

This looks great! Nice work @mschoneman !