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 stores the name like "hostname/type-typeinstance/plugin-plugin-instance" (https://collectd.org/wiki/index.php/Naming_schema). On my first attempt, influx wouldn't let me create a series with / in the name but that might have been user error. It seems like I can store it in that format now. I'll update this PR when that is ready.

I'm open to other formats for the name as well.