Require units for CCDData objects and update tests
Require metadata to be an instance of fits.Header to get a case-insensitive dict
Add fail to tests to work around problem with astropy
Remove unused imports and unused code
Use astropy.io.fits and registry to handle fits read/write
Move coverage information from .coveragerc to the astropy-standard location

That location is ccdproc/tests/coveragerc
Document FITS read/write methods
Add Keyword class
Add copy method to CCDData
Modify create_variance to match API
Remove test that is now incorporated in a pytest parametrize
Implement and test API for specifying bias section
Expand documentation of section functionality
Add trim_image to ccdproc
Add subtract_bias
Allow overscan to be specified with a CCDData object or a fits_section
Allow trim_image to accept either a view or a fits_section and add more tests
Add dark subtraction
Use built-in astropy units where possible

Closes #15
Make the electron unit singular instead of plural

Closes #39