add ruby 2.1.2 to travis config
Add server-side support for channel multiplexing
refactor how we send messages to the client via websockets
use Rack::Utils##parse_query in enable_multiplexing?
coerce subscriptions into Array in MultiplexedHandler#subscribe_multiplexed
move Websocket::Handler#suscribe down for consistency with MultiplexingHandler code layout
add MultiplexingHandler#unsubscribe
use special request path to enable channel multiplexing