fix axes_grid for offsetbox api change
refactor draggable legend to support annotation
fix some issues in the bbox after the postscript distiller is run
added state to legend toggle
added drabbable state to legend -- thanks Adam Fraser
experimental support of Image._image_skew_coordinate
Support setting zorder keyword for ContourSet.
Fixed a bug in hist with stepfilled and log=True
Add units module to documentation
Fix units support for contour and contourf
