Adding redirect() stub to Application class

A small change, just to add a redirect() stub to the Application class, to prevent things like extension:install augering in if the extension does a $app->redirect() at the end of a custom install script.  The redirect() throws an exception, which I added a catch for in ExtensionInstallFile.php, as that's the command I'm working with at the moment.

PS, don't know whay the tab formatting is weird in the github code, it looks fine in Eclipse!