Shimmie Extension - User Groups

Split off another extension I was writing, which needed a method to group user to allow it to act on many users at once.

User Groups does that, and later extensions can add additional features to them, such as chat, forums or news. If they do become more social contructs like that, I'll have to add a few features to facilitate that, since right now only admin can create groups and add users to them.

Probably some manner of group ownership and a few settings the owners can change regarding membership will be next.

Anyway, the initial point was accomplished: groups of users.

GitHub Repository
Commit at time of post: e81093d3
Dependencies: Simple Extension