Add PixelAperture and SkyAperture to __all__
fix get_phot_extent link
Merge pull request #2 from bsipocz/docs_fix_links_II

fix get_phot_extent link
Keep area() as non-abstract method
Merge pull request #153 from larrybradley/aperture_imports

Add PixelAperture and SkyAperture to __all__