added tests for image comparison
Merge pull request #41 from anizami/new-image-tests

code for generating and comparing images
Make coord_type_from_ctype return a default coord_wrap
Add coord_type_from_ctype tests.
Merge pull request #19 from Cadair/coord_wrap

Make coord_type_from_ctype return a default coord_wrap
Compute tick angles by advancing a small amount in axes coordinates

This handles cases when a plot extends over a small number of pixels in
the WCS projection, but a large distance or number of pixels in display
coordinates. Addresses #45.