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/Utility/MkvFileIterator.class.php

@@ -0,0 +1,9 @@
+<?php
+
+class RippingCluster_Utility_MkvFileIterator extends FilterIterator {
+    public function accept() {
+        return preg_match('/\.mkv$/i', $this->current()->getFilename());
+    }
+}
+
+?>