udp json listener

(This is not ready to be merged as is)

I want something lighter than http for sending data to my influxdb and since my influxdb server and the producer are both on the same host I started with this simple listener which take UDP packets in with the standard json used for the http listener.

I want to know if this could be merged before working more on it (it may be simple but since I don't know much of go so it took me some time to even reach that point), it should works for simple use cases:
- I don't care about lost packets (looks like this can still happen even on loopback interfaces if packets are sent too quickly)
- packets misorder should not be an issue
- I think there is no limit on datagram size on loopback interface but I cant' find any evidence of this, does anyone know ?

So far this works but there is no config part and I still need a way to tell in which series the data will be written (currently hardcoded).

So.. what do you think ? Could it be merged ? (if I add the missing part namely the config and a way to tell which serie to write to)

(This interface is only meant to write data, obviously)