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 more informative error message when lat_0 is nonzero for projection='geos'
Add units module to documentation
Fix units support for contour and contourf
