tests - Fix deprecated warnings with unit tests

* default_scope with a hash is deprecated. Using lambda containing a scope.
* The app doesn't support multiple locales right now. Hence, setting i18n.enforce_available_locales to false

Related to #64.
fix(CSRF) - "Rails can't verify CSRF authenticity"
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.