Add Wercker support

Wercker [do not provide public api yet](http://devcenter.wercker.com/articles/faq/#toc_6) so you can call it a "temporary workaround". Because of that build status is only available via Project ID (see example).

P.S. I wasn't able to test the code locally because of dependency hell (i really don't need all that stuff), so we need see what Travis have to say.

Resolves badges/shields#265