Expose pageChange idle detection to application code

[djbeaumont]

It would be very useful for application code to implement loading of assets or start small pieces of computationally intensive work without blocking the critical rendering path. For example:

window.barometer.onPageIdle(function() {
  loadGoogleMapsApi();
  performLargeButNotCriticalAjaxRequest();
});

[msaspence]

This is a nice idea but it seems like it is outside the scope of barometer? Should this not have its own library and be part of it own library?

[theninj4]

In the perfect world we could break this project down into lots of micro-modules, ultimately we'd like this functionality available to us in production very soon - we've got a good implementation in a small module that we're prepared to load before everything else, we should simply use it.

[theninj4]

This is being renamed to onPageChanged and we're adding another callback for oncePageLoaded which will only trigger once.