Update protobuf definitions and refactor message receive and decrypt
codepath to support new protocol, including various flavors of sync
messages (sent messages, contacts, and groups).
Also cleans up background.js and lets libtextsecure internalize
textsecure.processDecrypted and ensure that it is called before handing
DataMessages off to the application.
The Envelope structure now has a generic content field and a
legacyMessage field for backwards compatibility. We'll send outgoing
messages as legacy messages, and sync messages as "content" while
continuing to support both legacy and non-legacy messages on the receive
side until old clients have a chance to transition.
Encapsulate the websocket resources and socket setup process in a
friendly OO class. The MessageReceiver constructor expects an instance
of EventTarget on which to fire message events asynchronously. The
provider of the EventTarget can then add/remove listeners as desired.
