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.
collectd ships with a default file (https://github.com/collectd/collectd/blob/master/src/types.db). If the user needs additional/different types, they can modify the installed file but the recommended way is to create additional types.db files and configure collectd to read them as well (via setting in the config file).