Removed some obsolete code for handling tag_svn_revision
This adds a check on the validity of the README and CHANGES rst, per the request in #196. Also removes some obsolete code for checking tag_svn_revision (this will have to be kept in for the older release branches, however).
This should fix the issue with the version/date regexp.
Should fix the release_version issue. Turns out I just had a typo in the entrypoints map.
Uses the [zest.releaser] section in setup.cfg instead of entry_points.  This of course requires some fairly recent version of zest.releaser (I forget exactly what version this was added in) but I'll figure that out and document it once I get around to documenting the release procedure.
Started on the developer documentation for PyFITS (most of it just links back to Astropy's developer guide which applies more or less to pyfits as well) and added what I think is a mostly accurate release procedure (though I'll have to do the next release from my own documentation and take notes on how well it actually matches up).