fix(lib/angular-scenario.js): Support manually bootstrapped apps

Recognize element decorated with karma-app attribute (and various data-*/x-*/etc
flavors) or class name as the element where the angular app will be manually
bootstrapped. Allow karma to run when no element has been decorated with ng-app.

I believe this behavior is known:

https://groups.google.com/forum/#!msg/angular/d9-QYgSeib8/Ziy4kTvndsIJ

It seems that the recommended workaround is to do something like `angular.element(document).unbind('DOMContentLoaded');` right after loading the angular lib. This is not exactly a convenient/elegant workaround to say the least and I'd like to propose an alternative.