Add a new scheduler parameter for control pending jobs size

[asvetlov]

await scheduler.spawn(...) should wait if too many pending jobs are scheduled.

[ebeseda]

From your comment to PR #16 : spawn_nowait should raise an exception if too many pending jobs.

Seems like you want self._pending act like a standard asyncio.Queue
where await q.put(item) waits for a free slot and q.put_nowait(item) raises QueueFull exception.

Is it necessary to use deque for self._pending ?

[asvetlov]

Technically no. asyncio.Queue with limited size should work fine.

[asvetlov]

Fixed by #24