Fix Python 3.x syntax
Implemented arbitrary direction tick labels
Added Elliptical frame, and factored out common code to frames. Also allow arbitrary frame class to be specified. Finally, use the frame class to define the axis patch.
Renamed update_limits to update_spines
Added initial documentation for custom frames and fixed bug
Improve wording of custom frame section
Added Hexagonal frame example
Added unit test and fixed bugs. Added ability to specify a dictionary for minpad.
Merge pull request #60 from astrofrog/non-rectangular-frame

Non rectangular frames
Minor fix