Adds Httplib and decouples Websockets, Http, et al
Adds Httplib and decouples Websockets, Http, et al

- Changes the request and client handling API to use new functions
  `handle` and `is_websocket_handshake` that dispatch on the handler.
- Http exposes a WebsocketInterface abstract type with default
  implementations of these functions that throw errors.
- Websockets implements these for a WebsocketHandler, which it defines.
- In addition, generic stuff like Request, Response, Headers, et al.
  have been moved to an Httplib module.

jiahao/WebSockets.jl,JuliaWeb/WebSockets.jl,JuliaWeb/WebSockets.jl