Add processing hook

A Processor is a process that recieves samples as they are parsed by the servers and performs actions on them before handing them over to the clients.

If a callable is defined in the 'processor' configuration variable, a Processor process will aply this callable to the sample recieved (host, name, val, time) and expects back a tuple of the same structure to forward to clients, or None to drop the sample.

This makes it easy to add all sorts of custom filtering/modification on samples.