Sitecore 6.5-7.0 Modal Dialog browser specific fix

Update to code to use jQuery Dialog for newer browser and old
showModalDialog method for Internet Explorer
Merge pull request #1 from jammykam/browser-specific-modal-fix

Sitecore 6.5-7.0 Modal Dialog browser specific fix. This works around issue in Internet Explorer not rendering the jQuery UI Dialogs correctly so IE uses the old showModalDialog function.