Improve esdoc's coverage report

[mbernier]

Issue Summary

Adding documentation is a great way to save yourself and other developers time when they are looking at code to understand what was coded or when they are troubleshooting an issue.

Steps to Reproduce

1. clone this repo git clone https://github.com/sendgrid/smtpapi-nodejs.git
2. run npm install
   a. If Add esdoc support #50 is not complete or merged yet, npm i chai@2.3.0 esdoc esdoc-coverage-plugin
3. run ./node_modules/.bin/esdoc
4. Open the coverage.json file that was generated in the docs directory
5. Find files that need more docs coverage and add it in!