Authentication added

I chose [devise](github.com/plataformatec/devise) to implement authentication.
I chose [rspec](https://github.com/rspec/rspec) as the test framework.

- The user is able to sign in/up with Facebook, Twitter or manually. (missing Linkedin only)
- When registering with a social network, a password is being generated but not sent to his email (Twitter doesn't return the email).
- When registering manually, a confirmation email is being sent.
- The user can have multiple social networks with the same e-mail - if the SN is different but the email is the same, a new oauth is added to the user.
- All users can have many social networks - Their social network's authentication token is being saved. (The permission to post on his wall must be configured on the app)

I have created temporary apps and tested all methods. All working.

Still, i'm looking for sugestions of methods to test the `CallbacksController` and the `User` methods.

I have open this pull request so we could start to discuss. Later i will add Linkedin and test the missing methods.