Don't force sliders to get evaluated on load

Since the load and play progress sliders are guaranteed to start from zero, set that through CSS. Calling Slider.prototype.update forces a re-flow because element dimensions are queried and style rules changed. That reflow consistently took around 60ms on my laptop which would mean dropped frames and "jerkiness" on initialization.

Here's a timeline from an iPad mini before this change:
![screen shot 2014-03-12 at 3 40 34 pm](https://f.cloud.github.com/assets/56667/2403234/525add50-aa2c-11e3-843f-62d6e189ffbf.png)

and after:
![screen shot 2014-03-12 at 5 23 47 pm](https://f.cloud.github.com/assets/56667/2403263/a124fa6a-aa2c-11e3-8d8f-b0b0aa582349.png)