Add timer support to JSDispatcherQueue

[vmoroz]

In this PR we add support for timers to the JSDispatcherQueue class.
The API design follows the DispatcherQueueTimer for WinRT.

Each timer submits to JSDispatcherQueue an instance of JSDispatcherQueueTimer.Job class.
It is added initially as a normal work item. So, that the timer job with delay 0 will behave the same way as any other JSDispatcherQueue task. If the time to execute the timer job is not expired yet, then we put it in a sorted job list.
Next time we look for new items we consider items from the job list.
We also consider the time to run next job when we start waiting for new work items.

In a follow up PR we will start using it for the hermes-engine example.