Remove Rack dependency.

[digitalextremist]

Somehow, we took the Rack dependent route on WebSocket support:

• https://github.com/celluloid/reel/blob/master/lib/reel/websocket.rb#L3
• https://github.com/celluloid/reel/blob/master/lib/reel/websocket.rb#L20
• https://github.com/celluloid/reel/blob/master/lib/reel/websocket.rb#L112

We need/deserve Rack independence completely.

[digitalextremist]

Closed in #189