Create repositories model

A repository is a representation of an acutal git repo that belongs to some user, it can be viewed, cloned, forked, pull from other repos and so on. This commit supplies the very basic infrastrucure for these features.
Extract username blacklist to: a common blacklist, a username blacklist, and a repo name blacklist (the last two contain the first)
Run simple_form generators
Hack Devise to work with strong_parameters

Devise doesn't do this right yet, so we subclass the passwords and registrations controllers and add the required param methods, as described here: https://gist.github.com/3350730