Added support for easing with liquid-box.

Hi,

Using ``` Ember.run.once(self, 'updateMeasurements'); ``` has definitely helped solve some of the issues I've experience with liquid-box but from time to time it still seems to collapse the elements completely during transition (using fade transitions) or disappear completely (during move-to transitions).

After spending some time trying to solve this bug I feel like the issues around measuring and the run loop are beyond my level of comprehension.

As a workaround it seems like using easing on the liquid box has reduced the frequency of the occurrences of this bug - and it also improves the effect of the animation IMO so I was wondering if you would consider including it in the library - it could be useful for others too?

Thanks again for your efforts, i'm amazed with the sophistication of the animations i'm now able to produce, it's really great!