Add powerdns input plugin

closes #614

plugins/inputs/powerdns/README.md

@@ -0,0 +1,68 @@
+# PowerDNS Input Plugin
+
+The powerdns plugin gathers metrics about PowerDNS using unix socket.
+
+### Configuration:
+
+```
+# Description
+[[inputs.powerdns]]
+  # An array of sockets to gather stats about.
+  # Specify a path to unix socket.
+  #
+  # If no servers are specified, then '/var/run/pdns.controlsocket' is used as the path.
+  unix_sockets = ["/var/run/pdns.controlsocket"]
+```
+
+### Measurements & Fields:
+
+- powerdns
+  - corrupt-packets
+  - deferred-cache-inserts
+  - deferred-cache-lookup
+  - dnsupdate-answers
+  - dnsupdate-changes
+  - dnsupdate-queries
+  - dnsupdate-refused
+  - packetcache-hit
+  - packetcache-miss
+  - packetcache-size
+  - query-cache-hit
+  - query-cache-miss
+  - rd-queries
+  - recursing-answers
+  - recursing-questions
+  - recursion-unanswered
+  - security-status
+  - servfail-packets
+  - signatures
+  - tcp-answers
+  - tcp-queries
+  - timedout-packets
+  - udp-answers
+  - udp-answers-bytes
+  - udp-do-queries
+  - udp-queries
+  - udp4-answers
+  - udp4-queries
+  - udp6-answers
+  - udp6-queries
+  - key-cache-size
+  - latency
+  - meta-cache-size
+  - qsize-q
+  - signature-cache-size
+  - sys-msec
+  - uptime
+  - user-msec
+
+### Tags:
+
+- tags: `server=socket`
+
+### Example Output:
+
+```
+$ ./telegraf -config telegraf.conf -input-filter powerdns -test
+> powerdns,server=/var/run/pdns.controlsocket corrupt-packets=0i,deferred-cache-inserts=0i,deferred-cache-lookup=0i,dnsupdate-answers=0i,dnsupdate-changes=0i,dnsupdate-queries=0i,dnsupdate-refused=0i,key-cache-size=0i,latency=26i,meta-cache-size=0i,packetcache-hit=0i,packetcache-miss=1i,packetcache-size=0i,qsize-q=0i,query-cache-hit=0i,query-cache-miss=6i,rd-queries=1i,recursing-answers=0i,recursing-questions=0i,recursion-unanswered=0i,security-status=3i,servfail-packets=0i,signature-cache-size=0i,signatures=0i,sys-msec=4349i,tcp-answers=0i,tcp-queries=0i,timedout-packets=0i,udp-answers=1i,udp-answers-bytes=50i,udp-do-queries=0i,udp-queries=0i,udp4-answers=1i,udp4-queries=1i,udp6-answers=0i,udp6-queries=0i,uptime=166738i,user-msec=3036i 1454078624932715706
+```