first attempt at refactoring CoordinateTransform
removed commented out code etc
Tick angles point along gridlines regardless of aspect ratio

I had previously computed the tick angle by advancing a fixed distance
in axes coordinates. This was wrong, because the axes coordinates always
run from 0 to 1, regardless of aspect ratio. The solution in this patch
is to compute the offsets by advancing in figure coordinates (i.e.,
pixels).

Fixes #62.
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
Merge pull request #63 from lpsinger/tick_angle_non_square_axes

Tick angles point along gridlines regardless of aspect ratio
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
Fix Python 3.x syntax
Implemented arbitrary direction tick labels