Bumpversion

fixes #177 
This is a proposal of configuring bumpversion inside django-filter.
With this tool the release process could look like this:
```
# X.Y.Z-dev -> X.Y.Z
bumpversion --tag --commit --list release

# build and upload a package or anything
python setup.py sdist bdist_wheel upload

# bump to X.Y+1.0-dev (without ``git tag``)
bumpversion --no-tag --commit --list minor

# push repo
git push ...
```

You can even try to automatically update ``CHANGES.rst`` in a similar fashion like in [bumpversion itself](https://github.com/peritus/bumpversion/blob/master/.bumpversion.cfg#L14-19) but I guess I want to leave this up to you.