Securely close SSL conection

For SSL in close_connection, current implementation use close_socket_gracefully to close socket firstly, then SSL_free. In my
testing, this approach may cause some pending write data missing. I'd suggest use standard SSL_shutdown in SSL conection. It
need call twice according to openSSL manual - one call sends the close_notify alert and one call receives the response from
the peer
Merge pull request #172 from jwang11/master

Securely close SSL connection by standard SSL_shutdown