Add logging proposals
Incorporate suggestions by @crawfordsm
Update user-specified keyword option to allow string or ccdproc.Keyword

This also updates the API for ccdproc.Keyword to allow string values.
Fix assert statements for first two options and logging so they will work
Initial attempt at image combination API
Clarify handling of offsets/transforms
Perform image combination with a Combiner class
Merge pull request #4 from mwcraig/image-combine

Initial attempt at image combination API