refactor setup.py get_version logic
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).

johnbachman/pysb,LoLab-VU/pysb,jmuhlich/pysb,LoLab-VU/pysb,alubbock/pysb,pysb/pysb,johnbachman/pysb