Fix support for datetime axes in 2d plots.

Fixes the error raisesd when 2d datetime coordinates are passed to
contour/contourf and the errors when 1d or 2d datetime coordinates
are passed to pcolor/pcolormesh.
Added a _timer_stop method to the Timer in MacOSX backend
Reorganization
bug: fix unit support for offsets in QuadMesh; closes #560
Merge pull request #2238 from efiring/quad_offset_bug

bug: fix unit support for offsets in QuadMesh; closes #560
Merge pull request #2233 from mdehoon/mac_timer_stop

Added a _timer_stop method to the Timer in MacOSX backend
Merge pull request #2199 from ajdawson/2d-datetime-coords

Fix support for datetime axes in 2d plots.
Merge branch 'master' of https://github.com/matplotlib/matplotlib