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)

Unfortunately not in this case : order matters and density arguments have to come before the input file in imagemagick. Otherwise, it could certainly be included as another associative array key.