Remove multipartform output stream

This branch removes the MultipartFormOutputStream (which is the last non-proprietary class in Catroid) and replaces it with the HttpBuilder-class (basically a leaner version of MultipartFormOutputStream).

This will also significantly improve code coverage in the web package.

Jenkins testrun:
http://catrobatgw.ist.tugraz.at:8080/job/Catroid-custom-branch-test/239/