Add support for infinite-scroll-delay attribute

infinite-scroll-delay specifies how many ms the infinite scroller should wait until causing the infinite scroll. After the delay, if the criteria for causing the infinite scroll are still met, the infinite scroller will scroll. This is useful if
you don't want the scroll to fire if a user just barely touches the bottom of the screen for a brief moment.

I struggled to get the tests to run in Testacular. Instead, I locally migrated the tests to use Karma and ensured the existing tests and my new test executed successfully (note, the migration to Karma is not included in this commit). I could submit another pull request with the Karma migration if you like.

I had to remove the jQuery wrapping $(container) calls in #handler because the unit test stubs for height would be ignored otherwise - this may have been an artifact of my local environment or Karma, I'm really not sure. However, the infinite scroller seems to work fine in browsers without the jQuery wrapping.