Initial attempt at image combination API

To be honest, I don't really like the details of the API proposed here because it requires the user to shuffle to much information from one step to the next. something like a ``Combiner`` class with methods for each of the steps that are currently functions might make more sense. 

I'm also not sure how much of the functionality of IRAF's ``combine`` we should provide. There are a large number of options in IRAF for rejecting pixels from a combination and I wonder how many are actually used in practice.

I've also completely sidestepped two issues:

+ Some way to specify IRAF's ``nkeep``, the minimum number of pixels to keep in doing a combination, primarily omitted at the moment because I'm not excited about the number of decisions IRAF makes automatically if the number of falls below ``nkeep``.
+ Logging of the decisions made in combining; this seems more complicated than the kind of logging described in https://github.com/crawfordsm/astropy-api/pull/3 


This is a starting point though...