Add a simple 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.
Add conf option to drop sample on Processor exception
Pass cfg to Processor's __init__
Add basic Processor tests
Add more tests for Processor

also improve existing, clean up code.
Update README.rst about Processor
Merge branch 'master' into processor

Conflicts:
	bucky/main.py
Make processor tests a bit more robust
Raise NotImplementedError properly in processor
Explicitly check for None in proc queue
regression fix: Gauges are constant until overwritten, and not reset at flush time.
Merge pull request #33 from dimrozakis/processor

Add processing hook