JsPoll: Manage timer events and invoke callbacks functions

This object can manage timer events and invoke registered callbacks functions.

It can start timer events that are triggered every period of a given time length. The timer events can be started an stopped any time.

The object can register and invoke callback functions to be called the timer starts, stops or on every tick of the specified timer period.

The list of invoked callback functions may be stopped if one of the functions returns false.