Handle url and descriptions from campfire

Campfire API seems to send url and description with some sounds
now leading to messages like

2013-03-07 23:51:28 48012  WARN CamperVan : WARNING: Could not set attribute 'url' to value 'https://123.campfirenow.com/sounds/crickets.mp3' on Firering::Message object. It is likely the Campfire API has changed. Please report this! (https://github.com/EmmanuelOga/firering/issues)
2013-03-07 23:51:28 48012  WARN CamperVan : WARNING: Could not set attribute 'description' to value 'hears crickets chirping' on Firering::Message object. It is likely the Campfire API has changed. Please report this! (https://github.com/EmmanuelOga/firering/issues)

This should fix
Merge pull request #14 from katzj/sound-message-attrs

Handle url and descriptions from campfire