Refactor: Replaces config.js module with nconf config handling

## What

Rather than using a config.js file, this pull request uses [nconf](https://github.com/flatiron/nconf) to read a json-formatted config file. It sets default values in case the user does not add their credentials. Also adds a fix for virgo.js version in package file.

## Why

Existing index.js does not actually handle the case of a missing './config.js'. This PR does.

## How

- [x] Adds nconf to dependencies
- [x] Replaces require('./config.js') with nconf initialization
- [x] Updates readme and example configuration to reflect nconf's inputs
- [x] Changes version tracking designation for virgo.js in package.json to keep up with sweet changes upstream


this can be pulled up to identityClient.getToken(callback);