Implement an optional progressbar using astropy.

Squashed commit of the following:

commit e0a7eb01dfad050ff849b9be2921faf7cae006c8
Author: Simon Mutch <smutch@astro.swin.edu.au>
Date:   Sun Jun 9 11:08:06 2013 +1000

    Add missing import

commit 3ed76db21d47efed8bbf9d3054094ce478ef0bc3
Author: Simon Mutch <smutch@astro.swin.edu.au>
Date:   Sun Jun 9 11:00:54 2013 +1000

    Implement progress bar using astropy.