Auto-generated cython_numpy wrapper

I added a new wrapping experiment 'cython_numpy_auto' that adds cython code auto-generation for the same approach as 'cython_numpy'.

To test drive:
* Auto-generate the cython code by running `python cython_generator.py` in directory `cython_numpy_auto`
* Install the wrapper with `python setup.py install --user` or similar command.
* Run the test with `python test_erfa.py` in directory `tests`.

Note:
You must have the `erfa` sources installed next to `erfa-wrapping-experiments` for the auto-generation to work.

To Do List:
- [ ] Automate structure generation/conversion from `erfam.h`
- [ ] Generate full API wrapper from `erfa.h`
- [ ] Integrated the templating engine `jinja2`
Ah -- having gone and looked at the SOFA/ERFA code, I see what you're saying now, and I see why that's the only real alternative.  Not to sound like a curmudgeon, but SOFA is totally doing it wrong.  The .h files should include the documentation, not the .c files.

At the end of the day, it probably matters little, because I suspect we would generate the `.pyx` file, and its resulting `.c` file through Cython, and ship that in our tarball, so the end user won't need the SOFA/ERFA `.c` files to build astropy.  But it might be worth a note to the SOFA folks about this.