Allow use of environment variables instead of using config.json

If you want to use this in a service like Heroku, keeping `config.json` in your repo is not desirable.

Modified `config.py` to check for certain defined environment variables that map to to the required options in `config.json`.

If all of the environment variables are defined, use the information from the environment variables instead of `config.json`

If not all of environment variables are defined, try to read `config.json`.