Websocket handling based on the callback path & fixes

- when the function evhtp_set_cb() is called with a path prefixed with 'ws://',
  this informs the library that this path can be weboscket compat if the client
  requests it. Note that the 'ws://' is cut off from the path before it's set as
  a callback.

- Fixed an issue where evhtp_ws_parser would return the wrong number of bytes
  processed.

- Note that websockets is not fully supported as of yet, frame encoding still
  needs to be applied to send_reply-type functions.