Allow clients to add images dynamically

This is mostly an extract refactor where the individual slide creation logic is pulled into a makeSlide method. This is done to allow for the addition of an addSlide method that can be called by Slider clients. 

I'm currently using the functionality from my client code like this:
```
  // a user has just uploaded a new image that should be added as the first slide
  var slider = that.get('slider'); // this is an ember.js idiom to fetch the slider object 
  var slideElement = document.createElement('img');
  slideElement.setAttribute('data-src', imageURL);
  slider.addSlide(slideElement, true);
  slider.gotoSlide(1);
```