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/passenger/README.md

@@ -0,0 +1,138 @@
+# Telegraf plugin: passenger
+
+Get phusion passenger stat using their command line utility
+`passenger-status`
+
+# Measurements
+
+Meta:
+
+- tags:
+
+  * name
+  * passenger_version
+  * pid
+  * code_revision
+
+Measurement names:
+
+- passenger:
+
+  * Tags: `passenger_version`
+  * Fields:
+
+    - process_count
+		- max
+		- capacity_used
+		- get_wait_list_size
+
+- passenger_supergroup:
+
+    * Tags: `name`
+    * Fields:
+
+      - get_wait_list_size
+      - capacity_used
+
+- passenger_group:
+
+  * Tags:
+
+    - name
+    - app_root
+    - app_type
+
+  * Fields:
+
+    - get_wait_list_size
+    - capacity_used
+    - processes_being_spawned
+
+- passenger_process:
+
+  * Tags:
+
+    - group_name
+    - app_root
+    - supergroup_name
+    - pid
+    - code_revision
+    - life_status
+    - process_group_id
+
+  * Field:
+
+    - concurrency
+    - sessions
+    - busyness
+    - processed
+    - spawner_creation_time
+    - spawn_start_time
+    - spawn_end_time
+    - last_used
+    - uptime
+    - cpu
+    - rss
+    - pss
+    - private_dirty
+    - swap
+    - real_memory
+    - vmsize
+
+# Example output
+
+Using this configuration:
+
+```
+[[inputs.passenger]]
+  # Path of passenger-status.
+  #
+  # Plugin gather metric via parsing XML output of passenger-status
+  # More information about the tool:
+  #   https://www.phusionpassenger.com/library/admin/apache/overall_status_report.html
+  #
+  #
+  # If no path is specified, then the plugin simply execute passenger-status
+  # hopefully it can be found in your PATH
+  command = "passenger-status -v --show=xml"
+```
+
+When run with:
+
+```
+./telegraf -config telegraf.conf -test -input-filter passenger
+```
+
+It produces:
+
+```
+> passenger,passenger_version=5.0.17 capacity_used=23i,get_wait_list_size=0i,max=23i,process_count=23i 1452984112799414257
+> passenger_supergroup,name=/var/app/current/public capacity_used=23i,get_wait_list_size=0i 1452984112799496977
+> passenger_group,app_root=/var/app/current,app_type=rack,name=/var/app/current/public capacity_used=23i,get_wait_list_size=0i,processes_being_spawned=0i 1452984112799527021
+> passenger_process,app_root=/var/app/current,code_revision=899ac7f,group_name=/var/app/current/public,life_status=ALIVE,pid=11553,process_group_id=13608,supergroup_name=/var/app/current/public busyness=0i,concurrency=1i,cpu=58i,last_used=1452747071764940i,private_dirty=314900i,processed=951i,pss=319391i,real_memory=314900i,rss=418548i,sessions=0i,spawn_end_time=1452746845013365i,spawn_start_time=1452746844946982i,spawner_creation_time=1452746835922747i,swap=0i,uptime=226i,vmsize=1563580i 1452984112799571490
+> passenger_process,app_root=/var/app/current,code_revision=899ac7f,group_name=/var/app/current/public,life_status=ALIVE,pid=11563,process_group_id=13608,supergroup_name=/var/app/current/public busyness=2147483647i,concurrency=1i,cpu=47i,last_used=1452747071709179i,private_dirty=309240i,processed=756i,pss=314036i,real_memory=309240i,rss=418296i,sessions=1i,spawn_end_time=1452746845172460i,spawn_start_time=1452746845136882i,spawner_creation_time=1452746835922747i,swap=0i,uptime=226i,vmsize=1563608i 1452984112799638581
+```
+
+# Note
+
+You have to ensure that you can run the `passenger-status` command under
+telegraf user. Depend on how you install and configure passenger, this
+maybe an issue for you. If you are using passenger standlone, or compile
+yourself, it is straight forward. However, if you are using gem and
+`rvm`, it maybe harder to get this right.
+
+Such as with `rvm`, you can use this command:
+
+```
+~/.rvm/bin/rvm default do passenger-status -v --show=xml
+```
+
+You can use `&` and `;` in the shell command to run comlicated shell command
+in order to get the passenger-status such as load the rvm shell, source the
+path
+```
+command = "source .rvm/scripts/rvm && passenger-status -v --show=xml"
+```
+
+Anyway, just ensure that you can run the command under `telegraf` user, and it
+has to produce XML output.