Correct problem with over-escaped NUL for PLAIN auth

The plain AUTH was separating the username and password with
"\\0". These are two characters, 5C '\' 30 '0' .

This patch changes it to use "\0" which is a single character, 00 NUL.

It also includes a test script to try sending email with only PLAIN
auth.
Merge pull request #57 from evanp/authplain

Correct problem with over-escaped NUL for PLAIN auth