Include verbose service output in mail notifications

Extended the `service-notify-by-email` command to include [`$LONGSERVICEOUTPUT$`](http://docs.icinga.org/latest/en/macrolist.html#macrolist-longserviceoutput)

Notification emails will contain the full output as returned by a check, instead of just the first line. If used with `check_http -v`, this will include the HTTP response body of the failed check in the email notification.