Add factories

Currently one must know whether the repo is GitLab or GitHub, etc.

A factory function should take a remote name or URL and return a Repository object.

https://github.com/coala/community/blob/master/community/git.py contains logic based on http://pypi.python.org/pypi/git-url-parse which can help with this process, and I hereby grant MIT license for anyone lifting the code from there to put it into IGitt ;-)