added versioneer for versioning

[alorenzo175]

Versioneer is nice because you never have to (or forget to) update the __version__ string. To make a new release version, you just add a git tag. When changes are made after a tag, the git commit is indicated in the version, so it might be something like 0.3.0+17.g39a420e indicating the code is based on git commit 39a420e which is 17 commits after the 0.3.0 tag.

The versioneer code is kept in _version.py so that version.py can be left alone for backwards compatibility. This seems kind of redundant, but might help a few people.

[wholmgren]

Thanks, I've wanted this a few times recently. Add a note and your name to the 0.3.1 whatsnew.

[wholmgren]

For future reference, also see also commit 73e1df1