Registration tests 2

Setup mocking http requests.
Write tests for username availability.
The last commit contains some smelly configuration that is required to use ```$httpBackend```.
We should try to find a way around https://github.com/kbaltrinic/http-backend-proxy/issues/1#issuecomment-53507699.
[gulp-preprocess](https://www.npmjs.org/package/gulp-preprocess) maybe?