Adding optional rate limit support.

[andy-trimble]

Added an asynchronous execution method that will retry in the event of a rate limit event.

This solves the following issue: #169

[andy-trimble]

@thinkingserious @mbernier If this pattern is acceptable, I'll begin porting it to the other libraries.

[mbernier]

Oh man this is cool.

@thinkingserious what was the process in the other lib? Was it sendgrid-csharp that built this in?

[andy-trimble]

@mbernier Here's a potential Go implementation as well.

[thinkingserious]

In C#: sendgrid/sendgrid-csharp#369
In PHP: sendgrid/sendgrid-php#453
Convo started in Ruby: sendgrid/sendgrid-ruby#119

[thinkingserious]

Hello @andy-trimble,

Thanks again for the PR!

We appreciate your contribution and look forward to continued collaboration. Thanks!

Team SendGrid DX

[thinkingserious]

Fantastic work @andy-trimble!

If you get a moment, would you mind throwing an example in here? Thanks!