convert files a.k.a. process graphics 

### Requirements
This uses the *docsplit* gem (which I've added to the Gemfile), which requires the following apt-get packages (on Ubuntu, they are:)
* graphicsmagick
* poppler-utils and poppler-data
It's more functional if you have [additional software loaded](http://documentcloud.github.io/docsplit/).

### Limitations
* It only converts the first page of any document.  (This is just the first phase.  Phase two will process multiple pages via a delayed job as child content.)
* It uses /tmp for temporary files (imagine that!)
* No tests have been written (I'm not sure how we want to handle those)

This addresses ticket #230 [and #4].