Add support to test the processing model in PhantomJS.

Before I land this I'd like to convert over all the previous tests to use PhantomJS and split out the processing model JSON files from the parsing JSON files.

@humphd do you have any preferences for the directory structure for the split up tests. I have a couple of ideas:

1. We keep the current directory structure and just have two JSON files for each test. One would be like [testName]-parsing.json and the other would be [testName]-procModel.json.
2. We have another level of directory underneath each testing directory. One would be `parsing-json` and the other would be `processing-model-json`. I like this one as we may not want to run the processing model over every VTT file we have.