Updated Readme

This PR brings the library's docs in line a little closer to the current state of [SendGrid Documentation](http://sendgrid.com/docs/).

It mostly focuses on updating the stats documentation [50e1668fc8e80845f2b5fa76b3d3b7a7bd2643c2], however it further updates a couple links and other minor things [54ace3bea4ca4425e367d427bb86990ee6a9706b], finally, it updates Sendgrid to SendGrid (where it's not code related) [67ac91664151ad5ebccea36beff1ca4f79397d6e].

I tried to separate all changes so you can easily revert any you don't like. However, I'm happy to rebase it into one commit if you'd prefer.

Let me know if you have any questions or want me to make any changes.