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
adding do_rectangular_photometry function
adding do_rectangular_photometry usage to aperture_core
minor docs change
adding rectangular annulus aperture
remove check for zero lenght aperture side
Adding test
removing repeated lines
Fixing code and tests to pass tests
Simplifying error calculation to use find_fluxvar()
creating rectangular_overlap() function to simplify do_rectangular_photometry(); and updating RectangularAnnulus.do_photometry
Overriding method='exact' with a large subpixels for Rectangular* apertures