Integrate findobj into photutils

This massive PR merges my `findobj` package into `photutils` (and I was able to preserve the `findobj` commit history).

I placed the new files (`detection.py`, `findstars.py`, and `morphology.py`) in a directory called `detection`, but the functions are exposed at the `photutils` top level.  I placed `findobj/utils/scale_img.py` into `photutils/utils/scale_img.py`.  We can move things around later as needed.

@cdeil  I didn't reorganize any of the tests, but we can decide on the best approach later.