Enable in-process restarting

MediaBrowser.Common/Plugins/IPluginManager.cs

@@ -29,6 +29,11 @@ namespace MediaBrowser.Common.Plugins
         /// <returns>An IEnumerable{Assembly}.</returns>
         IEnumerable<Assembly> LoadAssemblies();
 
+        /// <summary>
+        /// Unloads all of the assemblies.
+        /// </summary>
+        void UnloadAssemblies();
+
         /// <summary>
         /// Registers the plugin's services with the DI.
         /// Note: DI is not yet instantiated yet.