refactored auto update endpoints to process Bitbucket's native POST payload

This changes the ```/api/bitbucket``` endpoint to only accept Bitbucket's native POST format, which looks [like this](https://confluence.atlassian.com/display/BITBUCKET/POST+Service+Management).

While I was at it, I simplified the code a bit. It works fine locally, but the tests fail (HTTP 405 issues an exception, so the assertion for testGithubFailsCorrectly is not executed [any more?]). Not sure what caused this failing test, though.