feat(login) - Use 'devise' gem for authentication

The authentication will be done using devise gem from now on. The reason behind using devise is that
it provides an easy way to authenticate during functional testing of controllers.

More info here:
https://github.com/plataformatec/devise/wiki/How-To%3a-Controllers-tests-with-Rails-3-%28and-rspec%29

Related to #64.