Fix MockInteractions.track to provide correct step values

[txsmith]

Fixes #59, the delta computations in the move function were the wrong way around.

Also get rid of the setTimeout call in the test suite since Polymer/polymer#3540 is resolved

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[txsmith]

I signed it!

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CLAs look good, thanks!

[txsmith]

Could you give an indication on whether this is likely to be accepted in the near future?