add ValidationErrorTemplate

this template uses a custom popop control to avoid some nasty issues
e.g. #71
- repositioning if host-window size or location changed
- repositioning if host-window gets maximized and vice versa
- it's only topmost if the host-window is activated
use ValidationErrorTemplate for TextBox
use error template for ButtonCommandMetroTextBox
Point NuGet installs to the QuickStart page
Merge branch 'master' of https://github.com/MahApps/MahApps.Metro
use MahApps.Metro.Native methods
use IsKeyboardFocusWithin to show error popup
AdornerDecorator for dates tab item
use ValidationErrorTemplate for DatePicker
remove ValidationErrorElement from checkbox and radiobutton
use ValidationErrorTemplate for PasswordBox
use ValidationErrorTemplate for ComboBox
ValidationToolTipTemplate is now obsolete
remove unnecessary margin
fix shadow effect
Merge pull request #798 from MahApps/punker76-validation-error-template-stuff

[RFC] use validation error template and fix some issues
Merge branch 'master' of https://github.com/MahApps/MahApps.Metro