Local Git archiving and Zenodo support

* Zip up the local Git repository using the 'git archive' functionality, so that PyRDM is not restricted to repositories hosted on GitHub. Note that if this fails, PyRDM will fall back to trying to download the archive from GitHub as a last resort.
* Implemented support for Zenodo.

Also:

* Use argparse rather than optparse.
* Renamed article_id to pid (publication ID), except in figshare.py to maintain consistency with the current Figshare API documentation, since the use of the word 'article' can get confusing when talking about publishing code.