API changes:

- Rename Commit._message to Commit.raw_message
- Rename Signature._name to Signature.raw_name
- Rename Signature._email to Signature.raw_email

New features:

- Remote.push(refspec)
- Tag.get_object()

And some bugs fixed.

Thanks to Brodie Rao, Fraser Tweedale, Andrew Chin and Carlos Martín
Nieto.