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 grid style from matplotlib.rcParams

Based on `matplotlib.axis.YTick._get_gridline`. Note that Matplotlib
implements gridlines with `Line2D`, whereas wcsaxes implements gridlines
with `PathPatch`. The two have slightly different option styles
regarding dashes, so we have to map between them.