Fix handling of px units in CSS hook setters.

turtle.animate { turtlePositionX: '+=100' } had been broken becuase
the CSS hooks for turtlePosition* were not handling things when
the units "px" were included in the set value.  This is now fixed,
and animate works again.  A test is added for this case.
Merge pull request #71 from PencilCode/hookfix

Fix handling of px units in CSS hook setters.