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
Forgot to remove _version.py from PEP8 check
Exclude _version.py from coverage report
Update changelog with versioneer support
Merge pull request #117 from leouieda/versioneer

Added versioneer support