Hidden flyout resize fix.

This fixes an issue where flyouts would get resized while hidden and
then getting partially shown.
Add support for menu's

Make regular menu's work in MahApp's metro
Merge pull request #297 from chandramouleswaran/master

Add support for regular menu's
Merge pull request #295 from lukaspj/master

Hidden flyout resize fix
Merge branch 'master' of git://github.com/MahApps/MahApps.Metro

Conflicts:
	MahApps.Metro/Controls/Flyout.cs
	MahApps.Metro/Styles/Controls.ContextMenu.xaml