transform PLOS thesaurus to JSON

Here's an [nbviewer link for the notebook](http://nbviewer.ipython.org/github/cmgerber/PLOS_Cloud_Explorer/blob/iss34/ipython_notebooks/plos_data_transform.ipynb) where I did the conversion. Check it out and see if the JSON is really OK. Note that I exported it to a new file, `plos_full.json`