Use versioneer to handle version numbers

versioneer (https://github.com/warner/python-versioneer) automatically
creates version strings based on git tags and the latest commit hash.
To create a new version, simply `git tag` and versioneer will update
everything when setup.py is called. When not working from a release (for
development), trackpy.__version__ will have:

* the last version
* the number of commits since the last version
* the latest commit hash
* 'dirty' if the repo has uncommitted modifications

e.g., `0.2.1-8-g608fc6e-dirty` for this specific commit.

This allows better provenance tracking since most research work is
probably done from `master` anyway.
Merge pull request #138 from nkeim/versioneer

BLD: Use versioneer to handle version numbers