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)

Isn't the default behaviour to use whatever is specified in the file with a fallback to 72? http://www.imagemagick.org/script/command-line-options.php#density I may very well be reading this wrong.