Fix 'stop button' flashing by tracking setTimeouts.

The interrupt('test') flag that is used to by the IDE to determine
whether the 'stop' button should show used to be able to return
'false' if the only pending animation was a setTimeout(fn, 0)
caused by an async continuation callback.  The result would be that
the "stop" button would flash (incorrectly) in a program that uses
a continuation-callback in a loop.

This change tracks those async delays and allows interrupt('test')
to stay "true" in those cases, so that the "stop" button is steady.
Merge pull request #49 from PencilCode/asyncpending

Fix 'stop button' flashing by tracking setTimeouts.