Change the version number from 0.4.1 to 0.4.2~git.

I've changed the release version number to 0.4.2~git rather than 0.5~git
because we don't know yet how long will it take to release 0.5.

Change-Id: I148f90dcc1520550bd09b8d03f150d4bec2e5a39
Add some comments in the C extensions and the python wrapper of arguments.

Change-Id: Ie83e4edcec76c462264a3023a691d87e8d14741f