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. 
Please catch the actual exception it will throw if you don't mind, this can prevent daemon shutdown amongst other things. It is very poor practice to do bare except like this, see PEP8 for ideas why.