ChemDoodle JSON and PubChem JSON formats

These changes add two new formats: [ChemDoodle JSON](http://web.chemdoodle.com/docs/chemdoodle-json-format) and [PubChem JSON](http://pubchem.ncbi.nlm.nih.gov/pug_rest/PUG_REST.html).

Both formats depend on [JsonCpp](http://jsoncpp.sourceforge.net), a public domain JSON reader and writer library for C++. This has been added as a single amalgamated file to the project, so it is not required as an external dependency.

I'm not sure how to provide unit tests and/or example files, but I can do so if needed.