Merge branch 'master' of https://github.com/sendgrid/sendgrid-go

Version Bump v3.4.0: sendgrid#96 Send a Single Email to a Single Reci…

…pient

Version Bump v3.3.1: Pull sendgrid#95 Use log instead of fmt for prin…

…ting errors

Version Bump v3.3.0: sendgrid#92 Inbound Parse Webhook support

Version Bump v3.2.3: sendgrid#91 Simplified code in mail helper

Version Bump v3.2.2: Pull sendgrid#80 Prism update

Version Bump v3.2.1: Added TOC to README, Added USE_CASES.md

Version Bump v3.2.0: Merge sendgrid#75

Version Bump v3.10: Added the ability to set a custom HTTPClient

Version Bump v3.0.6: Added troubleshooting section