Transport adapter contract and Long-Polling

I've refactored our WebSocket-only Channels into a "Transport Contract" to support any kind of transport protocol or `%Phoenix.Socket.Message{}` serialization format. This PR contains working WebSocket and Long-Polling transports, as well as an updated `phoenix.coffee/js` that includes long polling fallback. Instead of ripping up the current javascript client, I defined a `LongPoller` class that acts as a `window.WebSocket` shim. The long polling is automatically fallen back to if WebSocket's aren't available, and can be configured as an option, i.e.: `new Phoenix.Socket("/ws", transport: Phoenix.LongPoller)`. 

The current approach for the LongPoller on the server is to store the binary_to_term'd poller process in the conn's session and resume it for each poll. Messages are buffered by the `LongPoller.Server` until the client repolls and explicitly `ack`'s messages. The LongPoller controller also mounts itself at `/ws/poll` transparently. This will change when we tackle the `channel` macro options, something like `channel "rooms", RoomChannel, via: [LongPoller: [at: ...]]`. For now, I've left this aspect for later after we finalize the Transports.

The long polling still needs to be throughougly tested, and some docs need expanded, I appreciate any and all feedback. Thanks!

// @josevalim @scrogson @jeregrine @ericmj

TODO:
- [x] [phoenix.js] Remove `WebSocket.[CONSTANT]` references with `Socket.states` to fully support fallback.
- [x] [phoenix.js] Trigger close instead of error for non 200/204 response from polling
- [x] [phoenix.js] Only send heartbeat for WebSocket transport
- [x] [LongPoller] Fully unit and integration test LongPoller
- [x] [LongPoller] Add random salt to session when encoding pid 
- [x] [LongPoller] Add Server to application supervisor for clean shutdowns
- [x] [Socket] Deprecate `get_assign`
- [x] [Router] Do not allow `channel` to be used within scopes
Agreed. But it's between this and using dispatch options to route the websockets directly to cowboy's websocket handler, which is still entirely cowboy specific. With this approach, we at least to get avoid dispatch_options and stay in plug land. It also *might* be extensible to other adapters, but we don't have other one's to experiment with yet. I'm okay owning this bit of code until we can get a better solution in place. :)