Add influxdb plugin

[mark-rushakoff]

(Supersedes #424)

This was primarily intended to consume InfluxDB-style expvars,
particularly InfluxDB's /debug/vars endpoint.

That endpoint follows a structure like

{
  "httpd::8086": {
    "name": "httpd",
    "tags": {
      "bind": ":8086"
    },
    "values": {
      "pointsWrittenOK": 33756,
      "queryReq": 19,
      "queryRespBytes": 26973,
      "req": 428,
      "writeReq": 205,
      "writeReqBytes": 3939161
    }
  }
}

There are an arbitrary number of top-level keys in the JSON response at
the configured URLs, and this plugin will iterate through all of their
values looking for objects with keys "name", "tags", and "values"
indicating a metric to be consumed by telegraf.

Running this on current master of InfluxDB, I am able to record nearly
the same information that is normally stored in the _internal
database; the only measurement missing from _internal is runtime,
which is present under the "memstats" key but does not follow the format
and so is not consumed in this plugin.

$ influx -database=telegraf -execute 'SHOW FIELD KEYS FROM /influxdb/'

name: influxdb_influxdb_engine
----------------------------
fieldKey
blksWrite
blksWriteBytes
blksWriteBytesC
pointsWrite
pointsWriteDedupe

name: influxdb_influxdb_httpd
---------------------------
fieldKey
pingReq
pointsWrittenOK
queryReq
queryRespBytes
req
writeReq
writeReqBytes

name: influxdb_influxdb_shard
---------------------------
fieldKey
fieldsCreate
seriesCreate
writePointsOk
writeReq

name: influxdb_influxdb_subscriber
--------------------------------
fieldKey
pointsWritten

name: influxdb_influxdb_wal
-------------------------
fieldKey
autoFlush
flushDuration
idleFlush
memSize
metaFlush
pointsFlush
pointsWrite
pointsWriteReq
seriesFlush

name: influxdb_influxdb_write
---------------------------
fieldKey
pointReq
pointReqLocal
req
subWriteOk
writeOk