Input plugin for running ntp queries

see #235

plugins/inputs/ntpq/README.md

@@ -0,0 +1,60 @@
+# ntpq Input Plugin
+
+Get standard NTP query metrics, requires ntpq executable.
+
+Below is the documentation of the various headers returned from the NTP query
+command when running `ntpq -p`.
+
+- remote – The remote peer or server being synced to. “LOCAL” is this local host
+(included in case there are no remote peers or servers available);
+- refid – Where or what the remote peer or server is itself synchronised to;
+- st (stratum) – The remote peer or server Stratum
+- t (type) – Type (u: unicast or manycast client, b: broadcast or multicast client,
+l: local reference clock, s: symmetric peer, A: manycast server,
+B: broadcast server, M: multicast server, see “Automatic Server Discovery“);
+- when – When last polled (seconds ago, “h” hours ago, or “d” days ago);
+- poll – Polling frequency: rfc5905 suggests this ranges in NTPv4 from 4 (16s)
+to 17 (36h) (log2 seconds), however observation suggests the actual displayed
+value is seconds for a much smaller range of 64 (26) to 1024 (210) seconds;
+- reach – An 8-bit left-shift shift register value recording polls (bit set =
+successful, bit reset = fail) displayed in octal;
+- delay – Round trip communication delay to the remote peer or server (milliseconds);
+- offset – Mean offset (phase) in the times reported between this local host and
+the remote peer or server (RMS, milliseconds);
+- jitter – Mean deviation (jitter) in the time reported for that remote peer or
+server (RMS of difference of multiple time samples, milliseconds);
+
+### Configuration:
+
+```toml
+# Get standard NTP query metrics, requires ntpq executable
+[[inputs.ntpq]]
+  ## If false, set the -n ntpq flag. Can reduce metric gather times.
+  dns_lookup = true
+```
+
+### Measurements & Fields:
+
+- ntpq
+    - delay (float, milliseconds)
+    - jitter (float, milliseconds)
+    - offset (float, milliseconds)
+    - poll (int, seconds)
+    - reach (int)
+    - when (int, seconds)
+
+### Tags:
+
+- All measurements have the following tags:
+    - refid
+    - remote
+    - type
+    - stratum
+
+### Example Output:
+
+```
+$ telegraf -config ~/ws/telegraf.conf -input-filter ntpq -test
+* Plugin: ntpq, Collection 1
+> ntpq,refid=.GPSs.,remote=*time.apple.com,stratum=1,type=u delay=91.797,jitter=3.735,offset=12.841,poll=64i,reach=377i,when=35i 1457960478909556134
+```