Stop writing to install directory during tests

Also changes the --generate-reference option to require a directory to
save the generated images.
Keep result_dir in image tests if a test fails

Makes/deletes directories for each unit test
Nicer output on failures by throwing an Exception instead of asserting a
string
Rename and documention option for generating images for tests
Handle missing images in image comparision

Gives path for generating image for use in updating the baseline images
Save baseline image used in comparisions for volatile installs

This keeps baseline images when `setup.py test` fails
Fix generating images with setup.py

Requires some ugly monkey patching of AstropyTest
Also fixes typo that prevented distutils from accepting an argument for
generate-images-path
Merge pull request #105 from sargas/only-use-tmp-dirs

Stop writing to install directory during tests