[fix] mail: give required `-t` argument to sendmail

Previous versions of nodemailer.js used to add `-t` automatically to
sendmail command line, in order to fetch recipients from the `To:`,
`Cc:` and `Bcc:` fields. However, this has been recently removed so that
recipients can be specified on the command line instead. It is now
necessary for the caller to add `-t`, or sendmail will exit with error
75 because of the lack of recipients.

Compatibility with older sendmail versions are preserved as giving
multiple `-t` on the command line is similar to giving only one.
[fix] mail: do not use auth if no auth is requested

If the `auth` field in the email options is `{none}`, then no `auth`
should be added to the nodemailer.js options object. Otherwise,
authentication will be attempted with empty (and invalid) credentials.
Merge pull request #128 from samueltardieu/fix-email

Fix email (by Samuel Tardieu)