Add docs and tests for `fits2bitmap` script

[cdeil]

Looks like we have a fits2bitmap script in the master branch ... it should appear in the docs (currently doesn't, see here) and have some tests (currently doesn't, see here).

cc @larrybradley I think you added this?

[cdeil]

How to test output images against reference images was figured out in wcsaxes already (see here).
It would be nice if that became available in the Astropy core so that we can re-use it here.
But I think the logistics of where to store the reference images hasn't been figured out yet.
cc @astrofrog

[astrofrog]

By the way, APLpy has a well-tested make_rgb_image function which we could consider combining with what already exists in imageutils.

[cdeil]

@astrofrog Do you think this make_rgb_image function should be moved to imageutils or what's your suggestion?

[astrofrog]

Oh, I just realized fits2bitmap is only for single images (not RGB). I'll open a new issue to explain what I meant.