Basic Authentication

Implemented the 'POST request /token_authentications(.:format)'  that sends user account credentials and receives back an authentication token.

The authentication token is then saved in user preferences for future requests authentication.

![iOS Simulator Screen shot Apr 21 2013 7 51 57 PM](https://f.cloud.github.com/assets/910954/407108/0d4ce688-aadf-11e2-9669-3bfc84c4825c.png)