1.2.0 release

Changes:

• Reconnection APIs change, add transparent reconnecting feature
  • remove reconnect(std::string const& url).
  • Add set_reconnect_attempts, set_reconnect_delay , set_reconnect_delay_max ,set_reconnecting_listener, set_reconnect_listener for transparent reconnecting. All of them are optional to set values.
• Fix a bug of ignoring the first binary in the received message, reported in issue: #3