CircleCI Hook

On the basis of the [travis ci hook](https://github.com/gitterHQ/services/tree/master/lib/travis) i’ve created a similar hook for [circleci](https://circleci.com) which can be configured by adding the gitter hook urls in the `circle.yml` file. I’ve tried to include 3 unit-tests with a failing, canceled and successfull push which are working.

I’m not 100% sure about the contents of the payload in the test but hope they are the same in reality. If someone has better examples please add.