getcomposer.org URLLabel branding for installer and uninstaller

This experimental patch is adding a label with the text `getcomposer.org` (based on the AppUrl define) to the setup which can be clicked to visit the website.

![composer-setup-urllabel](https://f.cloud.github.com/assets/378849/105393/6791b8ea-69ac-11e2-9704-179dfcb91f04.png)