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
Merge pull request #149 from bsipocz/aper_adding_rectangular_annulus

Adding RectangularAnnulus aperture, and do_rectangular_photometry function
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