moved erfa auto-generation code to astropy/erfa
boilerplate updates
condensed cython_generator to one file

This is useful because it allows it to be used as a script from inside the source directory.  If it's in multiple files you have to make sure the whole subpackage properly imports, which could be a pain to deal with in some situations
more reorganizing to streamline generator
Adapt cython_generator.py to work with single-file erfa.c
updated template and setup to get erfa wrappers working
add warning to the top of erfa.pyx
update tests to actually do tests (and pass
updates erfa code to include semi-useful docstrings