Implement some missing APIs.

Implement Agent.set_query(key_id) for reverse lookup, and Trie.map(buffer) to load trie from an existing buffer in memory.
I wrote a script to test these features: https://gist.github.com/lotem/3aad0a2f77f990278cad

That's all I need for now. I can keep going writing my node app. I'll show you a demo when I finish, if you are interested! It's a cross-platform desktop app packaged with node runtime and all the needed modules, so I do not need my users to do `npm install` from the npm repository. Take your time for code review.

Thanks.