add plugin for Google Tag Manager

Added a plugin for Google Tag Manager and a test page to see it in action. The plugin follows the same model as the Google Analytics plugin, but pushes event and state information to Google Tag Managers dataLayer on pageviews or events rather than calling GA directly. This allows abstraction of tracking configuration (i.e. account ID, tracking type, etc.) from the application to an easy to use web UI at http://www.google.com/tagmanager