Implement events logging feature

Record events (i.e. pluginhook's calls) as syslog entries and
provide a shortcut command to display the last part of the log
file.

A brief summary of the changes follows:

dokku:
 - New DOKKU_EVENTS_LOGFILE envvar to hold logfile location.

plugins/20_events/commands:
 - events output log entries, a tail's follow-like mode is
   provided too.
 - events:[on|off] enables/disables logging.
 - events:list lists events that are logged.

plugins/20_events/hook:
 - Generic hook that writes log entries. All the events are
   registered via symlink to this script.

plugins/common/functions:
 - dokku_log_event() writes log entries via logger.
 - dokku_log_pluginhook_call() formats log entries and writes
   them via dokku_log_event().

plugins/20_events/hook

@@ -0,0 +1,5 @@
+#!/usr/bin/env bash
+set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
+source "$(dirname $0)/../common/functions"
+
+[[ ! "$DOKKU_EVENTS" ]] || dokku_log_pluginhook_call "$(basename $0)" $@