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)

Since PDF->image is usually vector rasterization, the density value is the DPI used by the Ghostscript delegate to rasterize the PDF. AFAIK vector PDFs hae no density value, although the images contained within them may.

Honestly, I've always made the assumption that IM renders at 72.0 by default but never really looked into it. From the PDF delegate it references a core default resolution:

https://github.com/trevor/ImageMagick/blob/master/branches/ImageMagick-7.0.0/coders/pdf.c#L404-L405

Which does appear to be set to 72.0

https://github.com/trevor/ImageMagick/blob/master/branches/ImageMagick-7.0.0/MagickCore/image.c#L122

So, I do think that 72 is the default.