An attempt at implementing basic support for websockets

The goal of this pull request is to give a shot at offering basic support for websockets, allowing to forward websocket requests to Cowboy websocket handlers.

It adds an `upgrade_to_websocket` function to connections to specify the handler module and the initial state to pass to `websocket_init`.

It also adds a `websocket` macro to the router DSL for a 1 line routing declaration.

Here is an example on how I use it [to interface with a pubsub server](https://gist.github.com/mathieul/6459540).

This is just a first draft to figure out if that's a direction you're interested in for the websocket support. Let me know what you think, thanks.