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.
Few questions on the typesdb:

1. What happens if it's not set, you don't seem to check it's value in server.go and looking at `collectd.TypesDb()` it will return an error if it can't open the file which will be an empty string in this case
2. How do users get that file is it part of the collectd installation. If so, then people need to install collectd in order to use this plugin which is sort of weird and defeats the purpose