Merge imageutils functionality into astropy core

This is a WIP, but is the PR to merge ``imageutils`` into the Astropy core package.

Notes:

* I have attempted to preserve the history but removed code that simply affected affiliated package infrastructure code.
* The branches and merge history *within* imageutils is lost since I rebased this, but I think this is actually less confusing this way. We just have a linear history for the package.
* I also updated issue numbers in the commit messages to always include ``astropy/imageutils``
* This does not include lacosmicx which can be added separately once the licensing is sorted out. Doing that separately will make reviewing this PR easier anyway
* It is marked as critical because this was assessed as 1.0-critical (not a critical bug though)

This is not ready for review or merging, and still needs updates to the docs, tests to pass, etc.

cc @larrybradley @cdeil @adonath @eteq @hamogu @ejeschke

Hi,
I wanted to give a try, but ` scripts/fits2bitmap` still use `imageutils.scripts.fits2bitmap` instead of  `astropy/visualization/scripts/fits2bitmap.py`, so it will not work.
Also, is there any reason why setuptools' entry_points is not used ? (it automatically generates the scripts)
https://pythonhosted.org/setuptools/setuptools.html#automatic-script-creation