Drive sending packets in JavaScript

Closes #8. This should fix a couple of issues:

1. For long subscriptions, we send zephyrs too fast. Wait for HMACKs between
   each. Also gives room to wait on SERVACK like the real ZSubscribeTo if we
   like.
2. It seems sendto blocks sometimes. Send through node instead.

Also wrap ZGetDestAddr and switch some Number::New's to Integer::New.