Add input for receiving papertrail webhooks (#2038)

plugins/inputs/webhooks/papertrail/README.md

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+# papertrail webhooks
+
+Enables Telegraf to act as a [Papertrail Webhook](http://help.papertrailapp.com/kb/how-it-works/web-hooks/).
+
+## Events
+
+[Full documentation](http://help.papertrailapp.com/kb/how-it-works/web-hooks/#callback).
+
+Events from Papertrail come in two forms:
+
+* The [event-based callback](http://help.papertrailapp.com/kb/how-it-works/web-hooks/#callback):
+
+  * A point is created per event, with the timestamp as `received_at`
+  * Each point has a field counter (`count`), which is set to `1` (signifying the event occurred)
+  * Each event "hostname" object is converted to a `host` tag
+  * The "saved_search" name in the payload is added as an `event` tag
+
+* The [count-based callback](http://help.papertrailapp.com/kb/how-it-works/web-hooks/#count-only-webhooks)
+
+  * A point is created per timeseries object per count, with the timestamp as the "timeseries" key (the unix epoch of the event)
+  * Each point has a field counter (`count`), which is set to the value of each "timeseries" object
+  * Each count "source_name" object is converted to a `host` tag
+  * The "saved_search" name in the payload is added as an `event` tag
+
+The current functionality is very basic, however this allows you to
+track the number of events by host and saved search.
+
+When an event is received, any point will look similar to:
+
+```
+papertrail,host=myserver.example.com,event=saved_search_name count=3i 1453248892000000000
+```