Unique version reference

I'm not a big fan of having two different places indicating the version of the package. It always leads at some point to a release with a inconsistent version. That can be no fun.

For this reason, I moved the version code to a separate file and I am referencing that file from both the `/version` view and the `setup.py`.

Because `setup.py` can't assume to have a working bamboo version (obviously), I am importing the `version.py` file specifically there.

I've also added a fallback code to the git branch & commit request as those might simply not be present: if installed via pypy, git repo is not there and git binary might not be neither.

It should be a very neutral change but since I'm not finished setting up bamboo on my laptop, please wait for travis to confirm.