Fix crash when checking validity of a ReactiveValidatedObject which has public static properties

As discussed on the mailing list. I'm fairly sure this all works, although I don't have VS2012 so can't test that everything's right there.

ReactiveUI.PropertyExtraInfo.CreateFromType was finding all public
properties, then attempting to treat them all as instance properties in
CreateFromTypeAndName. It now filters its GetProperties call to public
instance properties only.

This commit also add a test which fails without the change to
Validation.cs.