Allow accessing backend name

Author: ids1024

pafy/__init__.py

@@ -10,6 +10,7 @@
 from .pafy import set_api_key
 from .pafy import load_cache, dump_cache
 from .pafy import get_categoryname
+from .pafy import backend
 from .util import GdataError, call_gdata
 from .playlist import get_playlist, get_playlist2