Initial work for killable build jobs

* Jobs should be terminated after a predetermined timeout.
* Jobs should be killable through the REST API.
Jobs can now be terminated at will and with timeouts.
Merge branch 'master' of github.com:devhub-tud/build-server into killing-and-timeouts
Terminating builds should now work properly.
Merge pull request #4 from devhub-tud/killing-and-timeouts

Initial work for killable build jobs