Use Versioneer for version management

[itamarst]

Fixes #20.

[wallrj]

Thanks @itamarst

Notes:

• Tested sdist builds with from local checkout, after adding a version tag and by downloading a git archive from github and the version seems to be correctly interpreted in each case.
• Checked that all the versioneer installation instructions had been followed.

Perhaps it would be nice to document that the project uses versioneer and how to release the project. But perhaps that can be done in a separate pull request.

I think this looks fine. Please merge if you're happy with it.