Updates to the old PyFITS FAQ to better serve as an 'astropy.io.fits' FAQ, as more people are starting out using Astropy.  Removed a few questions mostly related to installation that are already covered generally by the Astropy installation docs.
Also make some slight updates to the docs on warnings
Apparently the label 'warnings' was already taken (I think it shouldn't be but I'm not going to try to track it down right now).
Fix incorrect object reference
Add several FAQs concerning performance
Explicitly include Pygments in deps for doc builds; see https://groups.google.com/d/msg/astropy-dev/aC2qODD-Lrw/LIiMG3kTKeAJ
Merge pull request #2865 from embray/fits/faq-update

Updates the PyFITS FAQ to serve as the astropy.io.fits FAQ