Implemented 'Changelog to branch' extension

This extension allows to specify a branch to calculate the changelog against.
Reason: we have implemented a workflow, where the user branches are merged into the "stable" branch before build and after a successfull build, they are pushed back to the central repository.

So if you use this extension and set origin/stable as the changelog comparison reference, you will get only commits, that have not been successfully built til now.

Background was: if one user pulls from stable into his user branch, and then pushed back the changes, the changelog shows all commits from the pushing user, but also the changes he pulled from the stable branche. But these changes were already build in a previous build, so they were shown multiple times in the changelog.