Document SendGrid transport and support API keys

[vitorio]

The SendGrid transport works, but doesn't support API keys. This adds the transport to the README, and adds transparent support for API keys. Supply a username and password as user and key, or just an API key as key.

[codecov-io]

Current coverage is 85.28%

Merging #62 into develop will not affect coverage as of af76b40

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Review entire Coverage Diff as of af76b40

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[amcgregor]

Minor note, the final released version will use simpler configuration keys "user" and "key", instead of the current underscore-delimited ones.

[vitorio]

Thank you!

[amcgregor]

Yeah, apologies for not paying attention to this earlier. Notifications were disabled for me on this project for some reason, which I have now fixed.