A new module (RestUtils.hs) along with a rest authenticator

Hello Oz,

days ago I had the need of writing a rest authenticator (just a fancy name for an handler combinator) to be able to "access" some routes just if "rest authenticated" (e.g. if I had a particular token in my http header). I showed the code to Doug, which seemed to like it. I've integrated a bunch of his suggestions and created this.

I thought the best place is a separate module, but I'm free to change my mind if you don't feel the need of a new specialised module. The idea is to gather everything regarding rest utilities.

Using the authenticator is trivial; it's an handler combinator, so it yields an Handler, which means can be integrated with existing combinators we have. I've attached plenty of documentation in the patch which should really make the code and the rationale behind it straightforward.

Feel free to give me feedback.
I hope it's something you might find useful as well :)

Greetings from a (very hot) Rome,

Alfredo