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
Maybe add `import numpy as np` to the example?
I think this might be automatic for docstring testing (which I think isn't on yet for photutils), but IMO it's nice if examples can just be copy & pasted and work.