Nodemailer: type definitions for the SMPT-specific options

As documented here:
https://github.com/andris9/nodemailer#setting-up-smtp

Added examples from the above source as tests.

The original definition of NodemailerTransportOptions looks questionable
with its AWS-specific fields, but leaving it alone for now since I'm
new to nodemailer.

Once typescript 0.9 is rolled out with its string parameter matcher,
that should be used for specifying the type of options of the second
parameter when the first one has value "SMTP".