Increase quality of PDF previews by controlling density used to generate...

Increase preview quality by allowing user control of density used. Defaulting to 600 significantly improves the text rendering in PDF->JPG.

![screen shot 2014-10-06 at 8 25 22 pm](https://cloud.githubusercontent.com/assets/244894/4535283/c5e63184-4db0-11e4-8e2b-60f4004c0967.png)
![screen shot 2014-10-06 at 8 25 08 pm](https://cloud.githubusercontent.com/assets/244894/4535284/c90ad8c4-4db0-11e4-907a-2dd13e7558c5.png)

Can this be built up like the other args?