Added a _timer_stop method to the Timer in MacOSX backend
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