Fix check of input image type #16

Closes #16
Add several tests
Add coveragerc

This coveragerc loosely mirrors that in the main astropy repository; it should be replaced/updated when the changes to the affiliate package template are merged in to ccdproc.

Closes #5
Remove unused imports
Merge pull request #31 from mwcraig/tests-100-pct

This pull requests brings test coverage close to 100%