[WIP] Unify controller and integration tests

This pull request is for the following project: https://github.com/rails/gsoc2014/wiki/Ideas#unify-controller-and-integration-tests
@pixeltrix I'm not sure about some of my solutions so I would like to seek your opinion on them. As there are quite a number of changes to it I'll do it in parts.

In some of the controller tests, we manipulate the controller instance different such as setting of the flash message. Therefore, I adopted the solution above but I dont think it's the most ideal. At the current moment I've not thought of a new solution and would love to hear your opinion on this. 

Thanks in advance!