[Ruby SDK] Add support for raw messages

Summary:
Our REST API users can access the raw contents of a message using the /messages/<id>/rfc2822 endpoint. This diff allows the same with the Ruby SDK, using the following API

```raw_contents = message.raw.rfc2822```

I haven't written ruby in year so feel free to point constructs which aren't idiomatic. By the way, is it that common for ruby code to be metaprogramming-heavy?

Test Plan: Tested manually

Reviewers: dlitz, bengotow

Reviewed By: bengotow

Differential Revision: https://review.inboxapp.com/D734