perl plugin: Added support to dispatch notifications to Perl plugins.

perl plugin: Added support to dispatch notifications to Perl plugins.

Perl plugin may now register a callback of type Collectd::TYPE_NOTIF which
accepts a single hash argument with the following members:

  severity
  time
  message
  host
  plugin
  plugin_instance
  type
  type_instance

Severity may be any of the following values (exported by the "notif" tag):

  Collectd::NOTIF_FAILURE
  Collectd::NOTIF_WARNING
  Collectd::NOTIF_OKAY

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