Enable remote "fullscreen" command

This is really only pseudo-fullscreen, as the js fullscreen API doesn't
allow fullscreen without user interaction.

MediaBrowser.Model/Session/PlaystateCommand.cs

@@ -29,7 +29,11 @@ namespace MediaBrowser.Model.Session
         /// <summary>
         /// The seek
         /// </summary>
-        Seek
+        Seek,
+        /// <summary>
+        /// The fullscreen
+        /// </summary>
+        Fullscreen
     }
 
     public class PlaystateRequest