Really basic coverage support for test-runner
Merge pull request #231 from web-animations/coverage

Really basic coverage support for test-runner
Merge pull request #232 from dstockwell/dead-code

Remove parent arguments and dead code. Simplify abstract methods.
Merge pull request #221 from dstockwell/clean-up-testharness

Remove unused code and fix some stylistic issues in testharness_extetensions/timing
New Timing interface
Update tests to use iterationDuration in place of duration
Simplify cloning of timing
Make Timing mutable and calculated properties on TimedItem read only
Use TimedItem.iterationDuration in tests.
Use null internally for linear timing. Handle special cases in iterationCount, iterationDuration and activeDuration
Rename TimedItem.timing to specified
Clean up preset timing functions
Rename TimedItem.animationTime to _animationTime
Rename TimedItem.iterationTime to TimedItem._iterationTime