Merge pull request #172 from jwang11/master

Securely close SSL connection by standard SSL_shutdown
Squashed warnings on win32 in Lua code
Improved docstring for mg_read()
Exported mg_url_encode()
Make websocket_ready capable to handle Websocket HEADER section

In send_websocket_handshake, it prints all HEADER in fix manner(end with "\r\n\r\n"), but leave no chance for user callback to hanlde HEADER. Actually
in some instance, user need update response HEADER such as "Sec-WebSocket-Protocol" after negotiation. Current implementation has websocket_ready callback,
which should be the place to allow user update both HEADER and Content. Thus, I replace "\r\n\r\n" with "\r\n" in send_websocket_handshake, and let
websocket_ready to handle remaining stuffs.