Before I said I wanted to move towards a polling model but I think we'll
stick with address/subscribe since the Dark Wallet main dev strongly
thinks we need instant notification.

I've made changes now that subscribe to address is by a prefix. This has
2 main advantages:

* If you generate addresses to share the same beginning bits then that's
less subscriptions for you.
* More privacy (variable level depending on prefix).

The same mechanism will be used for stealth too.

fetch_history() still remains unchanged. More to come on that front.