StreamerPlugin: Use BDataIO as base interface

* There's no need to force the streamer plugin to use a
BMediaIO. This class is supplied to accomodate more smart uses,
but in certain cases it makes just things more complex. If a
plugin doesn't need to have an internal caching method, then it
will be simpler to implement a block-consuming BDataIO and let
the internal BMediaIOWrapper to deal with caching.

headers/private/media/StreamerPlugin.h

@@ -18,7 +18,7 @@ public:
 								Streamer();
 	virtual						~Streamer();
 
-	virtual status_t			Sniff(BUrl* url, BMediaIO** source) = 0;
+	virtual status_t			Sniff(BUrl* url, BDataIO** source) = 0;
 private:
 	virtual void				_ReservedStreamer1();
 	virtual void				_ReservedStreamer2();