Performance fixes.

testCountPlugin only counts tests after root test completes.
testCountPlugin doesn't re-count tests every time.
Test.isComplete is cached.
Faster implementation of addClass & removeClass - uses classList when available.
notificationPlugin checks for permission before displaying notification.

Simplified inspection code so it's similar to before the plugin refactor.
Skipped tests no longer counted.
notificationPlugin no longer breaks when notifications are not available.
Merge pull request #28 from soxtoby/performance-fixes

Performance fixes