2D background and rms noise estimation

One more feature for `0.1`..... 2D background and rms noise estimation.  This is a pure-python method inspired by SExtractor.  It has more options than SExtractor (even allowing a custom calculation), but obviously it's slower than SExtractor or @kbarbary's `sep` (the slowest step is the iterative sigma-clipping along one axis of a 3D array using `astropy.stats.sigma_clip`).

As a note, I discovered while writing this that SExtractor *always* uses the "crowed" field background calculation (despite what its user's guide says).