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
Inherit default tick style from `matplotlib.rcParams`

Note that Matplotlib's defaults dictionary contains default settings
only for `xtick.*` and `ytick.*`. Because there is not a fixed mapping
between WCS axes and Matplotlib `x` and `y` axes, the best we can do is
arbitrarily pick one axis from which to inherit defaults.

This is explained in the docstring.