perl plugin: Exported plugin_dispatch_notification() to Perl.

perl plugin: Exported plugin_dispatch_notification() to Perl.

This adds the following function to collectd's Perl API:

Collectd::plugin_dispatch_notification:
  submit a notification to collectd's notification mechanism

  arguments:
  notif - notification

A notification is a reference to a hash with the following members:
  severity => $severity (default: NOTIF_FAILURE)
  time     => $time (default: time (NULL))
  message  => $msg
  host     => $host (default: hostname_g)
  plugin   => $plugin
  type     => $type
  plugin_instance => $instance
  type_instance   => $type_instance

The severity should be any of the Collectd::NOTIF_* constants.

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>