Show a single loading bar for multiple synchronous requests

It is not unusual for several subsequent requests to depend on data returned from previous requests. By default, this causes the loading bar to jump for each request, and doesn't give the user any idea of the overall load progress.

This pull request allows an `$http` call to specify a `loadingBarShouldExpect` integer parameter. If this parameter is supplied, the loading bar will take the future requests into consideration, showing progress across the whole request chain rather than bouncing between 0 and 100% for each individual request.

The functionality is found in the few lines changed in `src/loading-bar.js`. The other changes are to show an arbitrary example of the feature in action (load the example and run the "Chained Example").