Simple security for TCP and UDP line receivers.

Addition of support for a simple plaintext 'key' in the line and UDP receiver services, the presence of this key acts as a simple means to prevent unauthorised or unwanted metric sources from using these services. For better security, please consider using AMQP.

I'm currently using this in a production environment. I opted not to mangle the metric path. 
The PEP8 pedant in me says no, but the pragmatist in me says yes. Both are correct python, so leave it as is. That being said, it you make the algo pluggable, you won't be able to do a direct import such as that.