Allow youtube_dl/__main__.py to be called directly

youtube_dl/__main__.py

@@ -1,6 +1,14 @@
 #!/usr/bin/env python
 
-# Execute with python -m youtube_dl
+# Execute with
+# $ python youtube_dl/__main__.py (2.6+)
+# $ python -m youtube_dl          (2.7+)
+
+import sys
+
+if __package__ is None:
+    import os.path
+    sys.path.append(os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
 
 import youtube_dl