Features:
* Now displays the number of mentions that the user has not seen in the
notification.
* When no mentions are outstanding, display which servers the user is
connected to, not the last message.
* When more than one mention is outstanding, display the names of the
conversations with new mentions, not just the last message received.
* Notifications of mentions are suppressed if you're in the conversation
at the time of the mention.
* Notifications of mentions automatically clear when you bring up the
conversation.
* Vibrate notifications now generate the user's chosen default vibrate
pattern, not a hard-coded one.
* Add ticker text to the notification that's displayed when the IRCService
goes into the foreground, instead of displaying a blank ticker.
To allow for all of this, the implementation moves most of the details
of generating the notification text into the IRCService, which now
exposes addNewMention() and notifyConnected()/notifyDisconnected()
methods instead of the lower-level updateNotification().
