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

Something I don't understand ... when I say `fits2bitmap -h` I see
```
  -e EXTEN, --exten EXTEN
                        FITS extension number of the image to convert
```
which is different from what's written here in the code (the option is called `ext`, not `exten` and the description is different).

I'm running the `fits2bitmap` I just installed using `python setup.py install --user` from this branch:
```
$ which fits2bitmap
/Users/deil/Library/Python/3.4/bin/fits2bitmap
$ cat `which fits2bitmap`
#!/opt/local/bin/python
# EASY-INSTALL-SCRIPT: 'astropy==1.0.dev10924','fits2bitmap'
__requires__ = 'astropy==1.0.dev10924'
__import__('pkg_resources').run_script('astropy==1.0.dev10924', 'fits2bitmap')
```
For some reason it's running this `main` function (not sure what it's running).

Anyways ... probably this is an installation issue on my machine only?
I'll try again tomorrow morning.