add changelog tool

I know you're going to be so amazed by my bash skills that I'm going to get linkedin endorsements left and right.
This is a tiny tool to generate the change log from the git log based on the merged pull requests (which we've bee doing manually so far). 
Just redirect the output to CHANGES.md and type in you github user and password when prompted.
In the future we may also add github issues that have been labeled with the same release and ensure the P/R are labelled accordingly.