Add support for "Trailer Posters" Screen Saver

These commits add support for the "Trailer Posters" screen saver option on ATV, as discussed in issue #37.

The application.js file required a large amount of restructuring; for whatever reason the ATV doesn't like it when the application event handlers and non-primitive variables are initialized outside of the application entry point function (onappentry). When the handlers/variables are initialized from the global file scope the request for the screensaver.json file is never made.

The PlexPosterScreensaver generator was just hacked together, as there seems to be very little documentation on how PlexConnect communicates with PMS, and even less on the actual PMS api. Currently it generates an image list for recently added movies, but could be configured to grab images from any server/website that the ATV has network access to.