Documentation Improvements

Preview:
http://www.stsci.edu/~lbradley/photutils/
Done.  I've caught the `RuntimeWarning` for now.  It's a harmless warning, but I'll look into fixing this in `astropy.stats.sigma_clip`.  The warning occurs when every pixel along the `z` axis in the 3D data is masked (i.e. the image array is completely masked in the Background mesh).