adding OAuth

[Ken Hornstein]

>Ah, that library was not listed on the OAuth web page.  But ... I would
>caution you that it seems like that library hasn't been worked on in 4-5
>years and it looks like it only implements OAuth 1.0 (the original XOAUTH
>mechanism), and GMail now supports OAuth 2.0.

To follow-up to myself ...

I see that the OAuth people developed RFC 7628, which defines a new
SASL mechanism called OAUTHBEARER which fixes the concerns I had with
XOAUTH2.  And that is supported by Gmail; if I was going to write some
code, I'd focus my effort on that as it probably has the best long-term
support prospects.

--Ken