ClockWatcher - simple wall clock stream.

For applications that like to run with wall-clock time.
Cleanup for comments

Move to just "watchClock(duration)", its cleaner.
Cleanup tests to use Fake[Awesome]Async library!
Reviewer notes
Remove return and just call _starTimer()
Support re-listening to the clock stream.

Support multiple listeners.
Support leaving and relistening.
Added more tests.
Removed close check since its not needed.
TODO: re-think "from" time and interval.
Re-word the description, make it clear this is isochronus
Update readme
Change anchorMs to anchor, its cleaner
Move, rename, cleanup.
Add another example to README
Merge branch 'clockwatcher' of github.com:jtmcdole/quiver-dart into clockwatcher