Implementation of ref, update_ref, create_tree, tree, create_blob

Hi,


here is a patch to allow users to create a commit directly to GitHub. I implements a couple of missing RAW API Calls.

I've been able to get a file, update it, push it as a commit, update the tree and the head reference (complete commit sequence according to http://developer.github.com/v3/git/ )

I thought it might be useful to others as well.

Cheers,