'metro' style for 'frame' control

**Description**

Allows users to use a `MetroWindow` + a `Frame` against a `MetroNavigationWindow` which cannot be instantiated in XAML (bit annoying but there's no fix : http://support.microsoft.com/kb/957231). The style has been taken from the one in `MetroNavigationWindow`, it's a 1:1 replica.

Now since `MetroWindow` is based on `Window` it does not receive navigation events, I've added an extension method that offers this feature, currently for OnNavigated and later for other types of navigations events.

Demo project has been updated to show the functionality: https://github.com/aybe/MahApps.Metro/blob/master/samples/MetroDemo/Navigation/HomePage.xaml.cs#L18

As of the commit https://github.com/aybe/MahApps.Metro/commit/a503a3ec2c99a2783d931f9492789638fda5fe13, I consider the feature as complete and open for enhancements.