Integrate PyFACT functionality in gammapy

This PR integrates the [PyFACT](http://pyfact.readthedocs.org/) functionality I find useful into `gammapy`.
Some things are heavily re-written (use astropy, clean up API or implementation, restructure), it's not a 1:1 translation.

- [x] Add a note giving credit to PyFACT
- [x] Convert `map.py` content
- [x] Convert `tools.py` content
- [x] Convert `pfmap.py` script
- [x] Convert `pfsim.py` script
- [x] Convert `pfspec.py` script
- [x] Convert `cta_response_root2fits.py` content
- [x] Convert docs

There's much more to do, but I want to merge this now, so I've created a new issue to continue with the PyFACT -> gammapy integration in the future: #78