Added Bitbucket changes hook, refactor and code sanitize for GitHub hook as well

Just a hook for BitBucket POST Service. I also added documentation about it and the security issues that are related to.

I did refactor the github hook as well removing some unnecessary code and fixing some inconsistences and PEP8 violations. This has been tested with git and mercurial.

This pull is related to #643 and http://trac.buildbot.net/ticket/2186