Added handleAppOpenURL as a PAGE.js callback

If an app is started by a URL it will call PAGE.handleAppOpenURL on the
root page if it's an HTML page, or handleAppOpenURL: in native.

appMenu asserts the call and a custom implementation is required, either
in HTML or native, same as notifications.
Merge pull request #42 from reachlocal/url_open

Added handleAppOpenURL as a PAGE.js callback