Added a MTPopupWindow delegate protocol and a property "use Safari".

The delegate protocol can be expanded but for now it defines two optional methods which are called to alert the delegate when the popup window is opened or closed.

The property "useSafari" is useful for those of us using MTPopupWindow to display local files but want to direct external links to Safari.

The test project was updated with a couple buttons to test the new features.

Thanks

Dylan