Implement JX.WebSocket

Summary: Ref T6559. Wraps WebSocket in a reasonable driver class which does event dispatch, some state management, and handles automatic reconnect.

Test Plan: In Safari, Firefox and Chrome, connected to a websocket server and sent messages back and forth. Terminated and restarted server, saw automatic reconnects successfully reestablish a connection on all browsers.

Reviewers: btrahan, joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Maniphest Tasks: T6559

Differential Revision: https://secure.phabricator.com/D11252