Simple su implementation

Sorry to bother again, I asked last week on irc how to add a "su" function,
I know it is not the best implementation but I am just learning clojure!

If this is really bad don't worry just say it, I'll be very greatful for your time
anyways.

So the commit goes like this:

This is a very simple su implementation, adds a switch-user that
expects a handler and a function that returns the new user based
on the request