Add GPL modules

MediaBrowser.Controller/Channels/IHasCacheKey.cs

@@ -0,0 +1,13 @@
+
+namespace MediaBrowser.Controller.Channels
+{
+    public interface IHasCacheKey
+    {
+        /// <summary>
+        /// Gets the cache key.
+        /// </summary>
+        /// <param name="userId">The user identifier.</param>
+        /// <returns>System.String.</returns>
+        string GetCacheKey(string userId);
+    }
+}