Bzip2 reading

Hi Erin,

This branch adds the ability to read bz2-compressed files (externally compressed .fits.bz2, NOT tile-compressed).

I did the #include "bzlib.h" and link to "bz2" libraries unconditionally because I think they're fairly widespread, but you may disagree.

I did NOT add writing bz2.  It's probably doable without too much drama, I just don't need it right now :)

cheers,
--dustin