Fixes #10 - Implement signal for successful cas authentication

### What is the problem / feature ?

The attributes returned by successful authentication with the CAS server are stashed on the request session, meaning that any code that wants to interact with those attributes has to be linked into the request/response cycle.

### How did it get fixed / implemented ?

Implemented a new signal `django_cas_ng.signals.cas_user_authenticated` which is fired on successful authentication by the CAS backend.  This allows for code that cares about attributes or any other part of the CAS authentication to be separate from view code.

### How can someone test / see it ?

Code mostly.  Also look at the README for info on how to use it.

*Here is a cute baby animal picture for your troubles...*

![6933762244_e0aec12df9](https://cloud.githubusercontent.com/assets/824194/5018592/4e935800-6a75-11e4-8665-cb028263585a.jpg)