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

Also removed a few questions mostly related to installation that are already covered generally by the Astropy installation docs, and made some small improvements to other existing answers, including improving links to the API docs.

This brings to mind that maybe it would be good to have a general Astropy FAQ.  This would be different from the "known issues" page which specifically addresses known bugs and shortcomings.  But a FAQ that addresses common usage questions both in specific submodules and across Astropy (how to deal with warnings, logging, config, etc.) might be handy.  Most *answers* could be short, and link to the relevant documentation.  But I think it should be possible to come up with a fair number of actual FAQs.
you mention in the paragraph below that the data is not read from the page cache in the benchmark, given the numbers and assuming the code is complete this is likely not correct. you need a really beefy system to sustain 2GiBi/s disk IO. Writting the data before each test (which timeit does not do, it runs 3 times and takes the best) will not mean the page cache is empty, assuming enough ram written data stays in the page cache. You need to explicitly flush it with the madvise memory call to do proper IO benchmarking.

I also noticed python fitsio uses cftisio 3280 which does not have my optimized byteswapping, in that case numpy is indeed faster.

Do you have a reference to the byteorder dismissal? I was also thinking about proposing a convention for little endian fits data to FITS. I think it would be very useful.