Metro popover support

Support for declaring a popover inspired by Github's and Twitter Bootstrap's popover functionality. Works great with Toggle buttons as per example (checkout other tab in app)!

**Note:** If used inside a tab panel, as used in demo, the tab panel that hosts the target control must have an AdornerDecorator on it otherwise the popover will not work correctly due to a bug\limitation in WPF's implementation (see: http://karlshifflett.wordpress.com/2008/02/19/wpf-validation-errors-disappear-inside-tabcontrol-when-switching-tabitems/).