Cleanup for app.php

• A subscription currency is required to create a new subscription
• I believe we need to require the `$app` explicitly
• Closing out the file with `?>`

cc: @chrissrogers 
I'd rather leave the closing tag off, as it's optional and by convention should be omitted to prevent accidental whitespace. Not really an issue here, but I think the style convention is important

ref: http://us3.php.net/basic-syntax.instruction-separation