first attempt at refactoring CoordinateTransform
removed commented out code etc
axis labels are now placed similar to matplotlib
cleaned up comments, unused code etc
forgot to update ticks_labels reference image
testing minpad on overlay_features.png
Allow metadata to be passed when passing transform instead of WCS to get_coords_overlay
Avoid duplicating 0 and 360 in coordinate range for longitudes
Added unit test for get_coords_overlay, with custom transform and coordinate metadata.
Make sure we either pass wcs or transform/coord_meta to CoordinateMap
Allow initialization of WCSAxes from transform/coord_meta
cleaned up some code
added documentation for minpad
Merge pull request #56 from anizami/frames

issue35 - add ability to use any coordinate system
Merge pull request #57 from anizami/padding

Change way the axis labels are positioned
Merge pull request #64 from astrofrog/overlay-meta

Better support for coordinate overlays
Make sure context gets reset
Plot degree labels in TeX mode

Use Unicode character or TeX math mode depending on whether `tex.usetex`
is enabled. Avoids exceptions like the following:

    UnicodeEncodeError: 'ascii' codec can't encode character u'\xb0' in
    position 290: ordinal not in range(128)
Add unit test