Merge pull request #165 from astrofrog/elliptical-overlap-grid

Cleanup of Cython code
Simplify how annulus photometry is handled
Fix doctest
Merge pull request #166 from astrofrog/simplify-annulus

Simplify annulus photometry
This moves get_phot_extent to aperture_funcs so as to avoid having to pass extents to the photometry functions - this will make it easier to use drop-in replacements for the core photoemtry, such as SEP