New SendGrid notification module

This is a new notification module that uses the [SendGrid web API](https://sendgrid.com/docs/API_Reference/Web_API/mail.html) instead of SMTP to send an email to one or more recipients.

The creation of this module came about while I was at SendGrid's offices on Thursday. I was hoping to use their service with the mail notification module during my AnsibleFest talk coming up on Tuesday. Unfortunately, using SendGrid is not currently possible either with their SMTP or web API service. PR #6667 deals with SMTP authentication. This pull request is different because the code makes an HTTPS call to the SendGrid to send the email. The module will be especially useful in heavily firewalled environments that block SMTP ports.