Turn warnings into $qpsmtpd->log() calls

This can be convenient, especially when one wants to emit a warning that will
show up when running the test suite but still be properly logged in production
Change qw() to qw[] in warn_level()
Add some documentation
Clarify doc
Fix unstable use of $1 in Qpsmtpd::warn_level()
Merge pull request #168 from jaredj/warnings

Turn warnings into $qpsmtpd->log() calls