Java Mail Helper Refactor

[thinkingserious]

Issue Summary

An interface to easily interact with the mail/send endpoint of the SendGrid v3 Web API.

The specification for this project is here and the discussion around this specification can be found here.

Acceptance Criteria

All of the use cases defined here are implemented and all corresponding tests are passing.

[Ronpgo]

Will this also deal 429s, i.e. sleeping the amount suggested in the response header if a rate limit exceeded?