kite: Export an HTTP muxer to add more HTTP endpoints.

Kite had only /kite websocket endpoint which the kites communicate
through.

This PR adds a new field, HTTPMuxer, to satisfy the need to add more
HTTP handlers, such as /healtcheck, /version endpoints.
kite: Update function comment.
kite: Introduce 2 new methods on kite.

HandleHTTP and HandleHTTPFunc methods enables us to add custom HTTP
handlers. HTTPMuxer interface is removed due to fatih's request.
Merge pull request #75 from igungor/http-muxer

kite: Enable adding custom HTTP handlers