User role actions

Provides extra actions to add and remove roles from users.

WARNING: HALF-BAKED MODULE!

This module is a work in progress for a client and as such as only written up to the point that I need.
In particular, the 'add role' action is not yet implemented.
There are a few TODOs in the code as well -- things that aren't as elegant as they could be.

I am releasing it in the hope that others will find it a useful starting point and take it a little further with patches :)