Merge pull request #149 from ChrisBeaumont/setup_refactor

Overhaul of installation
fixed poorly-written test
Switched from ATpy to Astropy for tables. Included FITS reader/writer for backward-compatibility with Astropy 0.2.
Added note about fits_io.py file
Make ASCII identifier work with Astropy 0.2 and 0.3.dev
Only register FITS readers/writers if they are not already registered
Remove TabularData and refactor uses in examples
Remove references to ATpy
Updated data tutorial doc
Removed remaining atpy references
Fill masked values with NANs
Updated .gitignore