Add phusion Passenger plugin

Gather metric by parsing XMLoutput of `passenger-status` utility.
More information of this utility:
https://www.phusionpassenger.com/library/admin/apache/overall_status_report.html

closes #522

plugins/inputs/all/all.go

@@ -20,6 +20,7 @@ import (
 	_ "github.com/influxdb/telegraf/plugins/inputs/mysql"
 	_ "github.com/influxdb/telegraf/plugins/inputs/nginx"
 	_ "github.com/influxdb/telegraf/plugins/inputs/nsq"
+	_ "github.com/influxdb/telegraf/plugins/inputs/passenger"
 	_ "github.com/influxdb/telegraf/plugins/inputs/phpfpm"
 	_ "github.com/influxdb/telegraf/plugins/inputs/ping"
 	_ "github.com/influxdb/telegraf/plugins/inputs/postgresql"