Add countTimers method

[thejohnfreeman]

Fixes #163 and #213 by adding a method to count the waiting timers. I (and I believe others: Facebook, @radarfox, @SimenB, ?) want this method to check for "timer leaks" in our tests.

[SimenB]

I like it 🙂

FWIW, if we document clock.timers I'm fine with that as a solution, as the helper added in this PR would then be trivial to implement in user-land

[radarfox]

Don't forget, that the clock.timers are not initialized for some reason, so the triviality is questionable, as you have to remember to do the empty check every time.

[fatso83]

Thanks for the work. Released as part of lolex 3.0