Added support for HiDPI displays

I added a bit of code from the pdf.js viewer which allows HiDPI rendering of the PDF preview pane. 

Exhibit A: ShareLaTeX sans Retina

![screenshot 2014-04-03 00 49 41](https://cloud.githubusercontent.com/assets/30054/2599748/8a8d2e10-baeb-11e3-8e5d-73f304576d13.png)

Exhibit B: TO THE MOON!

![screenshot 2014-04-03 00 47 44](https://cloud.githubusercontent.com/assets/30054/2599749/a44e9d48-baeb-11e3-87f5-a5b2ce51fc65.png)

It might be hard to tell the difference, being the epitome of first-world-problem, so here's the screenshots zoomed in a lot.

Without HiDPI support:

![veryzoom](https://cloud.githubusercontent.com/assets/30054/2599758/f7be135a-baeb-11e3-95ac-a042c6359de6.png)

With HiDPI support:

![screenshot 2014-04-02 22 18 54](https://cloud.githubusercontent.com/assets/30054/2599756/f1dee07c-baeb-11e3-96e0-41bfb57af80b.png)