Add minimal WebSocket server implementation for evhttp (#1322)

Add minimal WebSocket server implementation for evhttp (#1322)

This adds few functions to use evhttp-based webserver to handle incoming
WebSockets connections. We've tried to use both libevent and libwebsockets in
our application, but found that we need to have different ports at the same
time to handle standard HTTP and WebSockets traffic. This change can help to
stick only with libevent library.

Implementation was inspired by modified Libevent source code in ipush project
[1].

  [1]: https://github.com/sqfasd/ipush/tree/master/deps/libevent-2.0.21-stable

Also, WebSocket-based chat server was added as a sample.