Delegate Pull-Request Merging to Github 

As described in #35, I've removed the local git functionality in favour of the merging functionality provided by Github.

Besides from deleting code that's not longer needed, I've tried to keep the changes to a minimum. These are:

- I've removed the "testing_branch_name" from the configuration file
- The existing Jenkins configuration hasn't changed, but users will have to provide the Refspec for pull-requests (see Readme)

Tested this with our Jenkins setup and it works like a charm.