notify_email plugin: Add plugin to send notifications via email.

notify_email plugin: Add plugin to send notifications via email.

Hello, Collectd.

I wrote a new plugin - notify_email.

This plugin will notify you when threshold conditions are met - one email per
one threshold notify. Email can be sent more then one recipient. Plugin does
this by initiating SMTP session to specified SMTP server (it can also do SMTP
AUTH if needed).

Plugin uses libesmtp library.

Please note:
  1. SMTPFrom value sometimes is checked by SMTP server - you
     can get failure from SMTP server if you are using not-existent
     email.

  2. SMTPSubject can contain two '%s' - first for severity and
     second for hostname.

  3. Hostname used in SMTP connection - is hostname_g, hostname
     that is used for all collectd host identification. If it cannot
     be resolved by internet DNS - you can expect problems when sending
     emails.

Patch and new file are included as attach.