Make __main__ work in all scenarios with relative imports

Makefile

@@ -24,7 +24,7 @@ test:
 # TODO un-phony README.md and youtube-dl.bash_completion by reading from .in files and generating from them
 
 youtube-dl: youtube_dl/*.py
-	zip --quiet --junk-paths youtube-dl youtube_dl/*.py
+	zip --quiet youtube-dl __main__.py youtube_dl/*.py
 	echo '#!/usr/bin/env python' > youtube-dl
 	cat youtube-dl.zip >> youtube-dl
 	rm youtube-dl.zip

__main__.py

@@ -0,0 +1,6 @@
+#!/usr/bin/env python
+
+import youtube_dl
+
+if __name__ == '__main__':
+    youtube_dl.main()

youtube_dl/__main__.py

@@ -1,5 +1,6 @@
 #!/usr/bin/env python
-# -*- coding: utf-8 -*-
+
+# Execute with python -m youtube_dl
 
 import youtube_dl