Simplify API of photometry functions

**This relies on #165 and #166 which should be merged first, then this rebased**

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. I also vectorized the calculation of the extents.

To see the relevant changes here (without #165 and #166) see 19aa574367cb384e0fe0bdc4179b68429fe2dc89

cc @larrybradley @bsipocz @cdeil @eteq