Implement on.once and on.pausable

[kfranqueiro]

We've had a few discussions back and forth on this topic (largely connected to the discussion around extension events), and I did some fumbling around myself.

Ultimately, we have decided to implement once and pausable APIs largely as they existed in Dojo 1.x (though they would be implemented as separate exports of the on module in this case).

• They will accept the same arguments as on
• once will return an object implementing Handle
• pausable will return an object implementing a PausableHandle which should extend Handle with pause() and resume()