fix 2 bugs in the "Script" CultureConfigurationElement

In the previous commit/merge there are 2 bugs - 

 - The "CultureConfigurationElement" is using the wrong key to retrieve the "Culture" property, which results in a NullReferenceException
 - The attempt to convert from string to CultureInfo is wrong - the initial safe cast to string will always fail, which results in null being returned from the property.

additionally, I marked the Culture property "IsRequired=true" since it stands to reason that since the entire element is optional, if you're specifying the element the exact culture is required.  An empty string will gracefully fall back to Invariant.