Allow fluid width override & protect against a width smaller than elementWidth

I created a subclass of Ember.ListView that is fluid/responsive. The grid items expand to fill the row until there's enough room for the next column, and the width and height of the list dynamically update as you resize the browser.

In order to efficiently achieve this useful subclass, however, `syncListContainerWidth` needs to be overridable. This pull request provides the private `_syncListContainerWidth` for override, as well as ensuring that a list with a current width less than the elementWidth won't lead to divide by 0 js errors. I've added tests for the latter.