Clarify the payload for the commit API call

[davidascher]

It's not clear from reading https://gandalf.readthedocs.org/en/latest/api.html#commit what the format should be for the zipfile payload. I'm hoping it's not the entire repo that we want to see diffed/pushed.

[cezarsa]

I added a hopefully better description in ea16af7. In short, you don't need to send the entire repo in the zipfile. Only files to be added or modified should be sent in the zipfile. A downside of this approach is that it's not currently possible to remove files from the repository using this API call.