Added optional delegate method to MSNavigationPaneViewControllerDelegate protocol:
- (void)navigationPaneViewController:(MSNavigationPaneViewController *)navigationPaneViewController willUpdateToPaneState:(MSNavigationPaneState)state;

Notifies the delegate that the user has asked for a pane state change.  Passes the MSNavigationPaneState that has been requested.
Merge pull request #17 from giffnyc/master

Pull request - optional willUpdateToPaneState: method added to delegate protocol