Add flux-preserving mode to reproject

Continuing the discussion from https://github.com/astrofrog/reproject/pull/67#issuecomment-98984750 and #3, I think it would be nice if the high-level reproject API contained a flag `conserve_flux` (or something like that) that's `False` by default and when set does this ("sb" = "surface brightness):

``` python
solid_angle_image = compute_solid_angle_image(flux_image)
sb_image = flux_image / solid_angle_image
sb_image2 = reproject(sb_image)
solid_angle_image2 = compute_solid_angle_image(sb_image2)
flux_image2 = solid_angle_image2 * sb_image2
```

@astrofrog – Do you think this is a common enough usecase to expose in the high-level API functions or is there some better way?

Start by reading the discussion linked from pull request 67 and issue 3, then inspect the high-level reproject API and the compute_solid_angle_image and reproject entry points. Clarify whether conserve_flux should be exposed, keep False as the default, and define how the surface-brightness conversion and flux-preserving result should be validated.

Written by the indexing model from the issue text.