Added factorygirl gem + updated spec_helper

**This pull requests adds the factory_girl_rails gem to the gemset of this project, used to simplify mocking in tests.
The spec_helper.rb file has been edited, so factory_girl will be loaded and included in all specs automatically.**

FactoryGirl documentation can be found [here](https://github.com/thoughtbot/factory_girl/blob/master/GETTING_STARTED.md).

_Don't forget to do a `bundle install` after pulling this change._