enable plex for television (#73)

* add plex show, season sync

* sync plex episodes

* sync plex episode statistics

* update plex artwork as needed

* code cleanup

* add note about tests

ErsatzTV.Core/Domain/MediaItem/PlexEpisode.cs

@@ -0,0 +1,7 @@
+namespace ErsatzTV.Core.Domain
+{
+    public class PlexEpisode : Episode
+    {
+        public string Key { get; set; }
+    }
+}

ErsatzTV.Core/Domain/MediaItem/PlexSeason.cs

@@ -0,0 +1,7 @@
+namespace ErsatzTV.Core.Domain
+{
+    public class PlexSeason : Season
+    {
+        public string Key { get; set; }
+    }
+}

ErsatzTV.Core/Domain/MediaItem/PlexShow.cs

@@ -0,0 +1,7 @@
+namespace ErsatzTV.Core.Domain
+{
+    public class PlexShow : Show
+    {
+        public string Key { get; set; }
+    }
+}