(fits) File list downloader 

Both UKIDSS and FermiLAT return lists of matching FITS files, and I think in many cases IRSA does the same.  So, it makes sense to abstract out that step.

This downloader simply gets the URLs (using `get_readable_fileobj`) and opens them as `fits` objects, with an option of saving the file in an optional directory with a lot of optional file name arguments.  The intent is to make it easy to create files of the form:

`UKIDSS_G043.17+0.01_H2_GPS_12312012.fits`

or something along that line, rather than the cryptic form like:

`L130413170713F15B52BC06_PH01.fits`

(but still make it possible to keep that cryptic form in the filename)