Addresses two comments from the review: Mentions that astropy.io.fits can read/write gzip files (it can also read from .zip files but I ommitted that since it's not a very commonly used feature to my knowledge).  Also mentions support for HIERARCH cards.  I also added mention of some other interesting features of PyFITS that were previously ommitted--fitsdiff in particular.
Fixed a couple typos, and bolded new/modified text.
Merge pull request #27 from iguananaut/fits-edits

Addresses FITS-related comments from the review