Added MKV source/worker plugins, tidied sources page

Added placeholder for source plugin to read from an existing mkv file.
Added corresponding worker placeholder to transcode an mkv file using
ffmpeg.
Updated the sources page to show which sources come from which plugins.

lib/RippingCluster/Source/PluginFactory.class.php

@@ -31,7 +31,7 @@ class RippingCluster_Source_PluginFactory extends RippingCluster_PluginFactory {
         
         $sources = array();
         foreach (self::getValidPlugins() as $plugin) {
-            $sources = array_merge($sources, self::enumerate($plugin));
+            $sources[$plugin] = self::enumerate($plugin);
         }
         
         return $sources;