Support for Youtube videos

Hi!

I've worked on this by following the guidelines suggested on [this topic](https://groups.google.com/forum/?fromgroups=#!topic/catarse-dev/V-mtf6UPl9Q). Basically these commits allow users to provide video urls from Vimeo or Youtube for their projects. Validations are performed for both formats and the project page is loaded with the proper markup for the video iframe. I've added/upated some specs and examples in order to cover these changes in the project model, controller and decorator, so I think it's good to merge :smiley: 

Personally I consider Vimeo a lot better than Youtube, but I've been working on this because some people needed to work with Youtube videos (me included), and I think it's a nice feature to have on Catarse. Anyway, even if you don't consider that important, I'd love to have a feedback from you about my code.

Thanks!