Add custom url interception for beforeload

[eric-sofisoftwarellc]

If callback_url is specified when creating the BH instance, it can be intercepted before the webview loads it.
This tells us the auth step is done.

For example:
var BH = new BirdHouse({ ...
callback_url: "myfineapp://callback"
});

When this is found in the url for beforeload, we're done.