Input plugin: collectd

This plugin supports receiving metrics via collectd's "network" plugin. It parses the binary protocol and stores most metric data as columns. Values are all float64. It requires the types.db file to be on the influxdb system.

Feedback appreciated.
Sure. That section was pretty much a copy of the graphite section (https://github.com/influxdb/influxdb/blob/eef53f722892d98fdf30f8320bbdc3c1c0ac49ab/server/server.go#L138)