Add "ConfigToken", "AvatarURL" to bbb.JoinOptions

  https://code.google.com/p/bigbluebutton/wiki/081API#Updates_to_API_in_BigBlueButton_0.81
Replace LoadServerVersion() by LoadStringResponse(*http.Response, string) string
Add "DefaultConfigXML" and "SetConfigXML" methods to bbb.BigBlueButton

  https://code.google.com/p/bigbluebutton/wiki/081API#Updates_to_API_in_BigBlueButton_0.81

Type bbb.ConfigXML models "config.xml" as described in the BigBlueButton wiki:
  https://code.google.com/p/bigbluebutton/wiki/ClientConfiguration
Add WebSocket (and "/uh") handlers for "DefaultConfigXML" and "SetConfigXML"