WIP: merge imageutils 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

I think it will be worth looking into.  If part of the point of Astropy is to reduce duplication of effort and reinventing the wheel I think it's a bit much to have that going on within Astropy itself (there are already other examples of this).  

For the purposes of this package though it completely made sense, since modeling wasn't ready to meet the use cases in question, and it's not a particularly deep or significant effort in the first place (creating this simple transformation API, that is).  So I'm not suggesting any changes now--just that in the future I would be strongly in favor of looking into it.