Added sampling API docs

I've added API docs for the cython `downsample` and `upsample` functions, but they are not being picked up in the documentation by `automodapi`.  Any ideas @cdeil, @astrofrog, @eteq?

This is the API docs part of https://github.com/astropy/photutils/issues/73
Is there a short way of being more precise about what is done (dividing by number of pixels in the block in addition to replicating) than "replicating image pixels"?