Added versioneer support

This makes a version string with the number and commit hash (if the repo
is not a stable release) and puts it in `fatiando.__version__`.

See https://github.com/warner/python-versioneer

Fixes #112

Checklist:
- [x] Create/update docstrings
- [x] Make tests for new code
- [x] All tests pass
- [x] Can be merged
- [x] Changelog entry