Start an astropy_helpers package and initialize it wieht the setup_helpers and version_helpers modules from astropy; these are still heavily reliant on Astropy to work
Removed most requirements for astropy itself in the setup_helpers module, mainly by copying over a few utility functions (some of these utilities are no longer used in astropy as a result--should we remove them?)  Also removed any support for the astropy test and configuration frameworks, but that can be added back in later as needed...
Split the git utils from version_helpers out into their own git_helpers module.  This enabled creating a version of generate_version_py that can work without having Astropy or any other support libraries installed, and that also provides support for projects that don't use git (though they lose revision numbers in their version strings until and unless we add support for other VCS (I do already have code to do this for SVN and HG should we wish to add it)).