Major rewrite with unit tests to align with GitHub web hooks format

Hi, thanks for the module! I refactored it to be covered with unit tests (see [travis](https://travis-ci.org/nichtich/Git-Hook-PostReceive) and [coveralls](https://coveralls.io/r/nichtich/Git-Hook-PostReceive), to align with [GitHub web hooks format](https://help.github.com/articles/post-receive-hooks) and to be more documented. Would you mind updating the module although the payload changes are not fully backwards-compatible? I can also take over the module if you are not interested anymore.