Message history fix

 Expose way to clear history from MessageHistory

    * We don't necessarily want to always call ClearAll when using
      MessageHistory since it also removes the MessageTrackListener from
      each of the IMessagingHubs.
    * Expose ClearHistory (previously a private method called clearData)

@jeremydmiller I've got some tests around this, but are there any other tests you can think of that I should write for this?