Update MahApps.Metro/Themes/Panorama.xaml

The HeaderFontColor binding was not always working. And the problem and solution are described at http://stackoverflow.com/questions/4665645/cannot-override-labels-style-in-wpf
Update MahApps.Metro/Controls/Panorama.cs

Clicking a selected item should deselect it (i.e. toggle). This is particularly useful when the Panorama control is used to select multiple items.
Fixed GroupBox foreground color issue.
Merge pull request #2 from MahApps/master

FI Master -> Feature