Provide a meaningful __version__ attribute for param.

I've attempted to make versioning painless and automatic by using
"versioneer" (v0.9, from pypi). The version number is set
automatically based on git tags, so to release all you have to do is
'git tag' followed by 'python setup.py sdist upload' (I've added these
instructions as RELEASE.txt). Developers should also now get a
meaningful version number between releases.

Downsides:
- Adds a file to the distributed param package.
- New and untested method until we go through one release.