Merge pull request #8 from kiangtengl/ribbon-menu-support

Ribbon Menu Split Button Fixes
Fixed - Toggle button should have the same look when pressed both checked and unchecked
Merge branch 'ribbon-menu-support' of https://github.com/Tide/MahApps.Metro into ribbon-menu-support
MenuButton should not stay dark after the popup opens
It should go to the highlight color

However it does not go dark when you press it. It goes straight to IsChecked mode