Update Ocropus calls to work with v 0.7

I noticed the Ocropus functions the gem uses are no longer supported in the latest version. I basically just copied the four commands from the [FAQ](https://code.google.com/p/ocropus/wiki/FrequentlyAskedQuestions), although I added the `--minscale 2` flag since I think it should at least try to recognize low resolution images. Maybe this  could be part of the options hash though?

It works just fine as part of a script, but for some reason when I try to create a document object with an image file in IRB it hangs after returning the new object. I'm pretty sure its some problem with the image worker, but I haven't figured out what yet.