Simplify annulus photometry

**This relies on #165 and should not be merged until that one is merged and this is rebased**

Basically the idea is rather than do photometry twice and find the difference, we simply find the difference of the 'fraction' masks before adding up the pixels. This only possible with #165.

To see the change from this actual PR excluding the changes from #165, see 29ae6df775293c050639ed00400b9b56cbf6199e

The benchmarks show that this results in an improvement of around 10% to 50-60%:

Before: https://gist.github.com/astrofrog/78d52a572d60ef328b31
After: https://gist.github.com/astrofrog/0c841a0e5508864f168c

This will incidentally also make #161 a lot easier

cc @larrybradley @bsipocz @cdeil @eteq