Add fixture for CCD data and marker to set size
Add markers for scale and mean of distribution
Use fixture for ccd data instead of generating in each function
Refactor ccd data setup and addition of cosmic rays to test images

There were a number of smaller changes:
+ Cosmic ray flux increased to ensure all are caught given threshold
+ Change places where number of cosmic rays ways hard-wired to use number set at top of tests
PEP8 changes
Merge pull request #34 from mwcraig/ccddata-pytest-fixture

Add a pytest fixture to generate a test image
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