HTTP API

Continuation from Thursday/Friday's work with @fraenkel and @maxbrunsfeld.

Involves bumping to Go 1.3; uses the new ConnState API from `net/http`.

Decided to stick with basic chunked responses instead of WebSockets for Run and Attach; things get weird for Run with WebSockets because there's an initial request payload, and sending it over WebSockets felt weird as it sort of implies you can send it more than once.