refactor setup.py get_version logic

Old git-related code moved to get_git_version, and code previously inlined
in main moved to get_version. Also it now looks for the RELEASE-VERSION
file relative to where setup.py itself lives, so calling it from another
directory will now work (important for getting sphinx doc build to work
from an sdist).
include docs and tests in sdist

fixes #99
Merge pull request #107 from jmuhlich/improve_sdist

Improve sdist