avro

AVRO-1778. JavaScript: Add IPC/RPC support.

This commit adds protocols to the JavaScript implementation.

The API was designed to:

+ Be simple and idiomatic. The `Protocol` class added here is heavily
  inspired by node.js' core `EventEmitter` to keep things as familiar as
  possible. Getting a client and server working is straightforward
  and requires very few lines of code.
+ Support arbitrary transports, both stateful and stateless. Built-in
  node.js streams are supported out of the box (e.g. TCP/UNIX sockets,
  or even stdin/stdout). Exchanging messages over a custom transport
  requires implementing a single simple function.
+ Work both server-side and in the browser!