fixes #5
fixes #6

Fixes the mock TickerFunc to behave like the real one in that it won't run more than one callback at a time and will wait until the callback completes to return from a `Wait()` call.

Turns out #7 is bugged.  When we refused to call the TickerFunc callback, we didn't advance the next event time, so the Mock kept re-scheduling `fire()` and we could never finish processing on the tick time.