Add/Update Badges on README

[thinkingserious]

Similar to what was done on the SendGrid node README:

• put the logo at the top of the list
• Add License like so
• make sure the other badges are available like so:

![SendGrid Logo](https://uiux.s3.amazonaws.com/2016-logos/email-logo%402x.png)

For the following, make sure the correct repo is linked!
For email notifications, you can change the "nodejs" to the language of this repo (e.g. java, python, ruby, php, csharp, go, etc)

[![BuildStatus](https://travis-ci.org/sendgrid/sendgrid-nodejs.svg?branch=master)](https://travis-ci.org/sendgrid/sendgrid-nodejs)
[![Email Notifications Badge](https://dx.sendgrid.com/badge/nodejs)](https://dx.sendgrid.com/newsletter/nodejs)
[![npm](https://img.shields.io/npm/l/express.svg)]()
[![Twitter Follow](https://img.shields.io/twitter/follow/sendgrid.svg?style=social&label=Follow)](https://twitter.com/sendgrid)
[![GitHub contributors](https://img.shields.io/github/contributors/sendgrid/sendgrid-nodejs.svg)](https://github.com/sendgrid/sendgrid-nodejs/graphs/contributors)

Thank you and happy #Hacktoberfest!!