Add discovery group

[tjguk]

Add a simple (and optional) namespacing mechanism for discovery: any advertised name can be of the form / where informally represents related services.

The idea is that, eg, in a classroom setting where individuals or teams are doing the same set of work, you're likely to get confusion as they'll all be using the same names: "robot", "controler" etc. In this way, teachers can tell them to use TeamA/robot or Fred/robot.

Another use is where a federation of services wants to be discovered, eg, when farming out work to one or all of a number of workers or subscribers.

Support for this could be offered in two ways:

• Adding a "discover_group" which will returns all the services with that group prefix
• Adding a "group" parameter to the existing .discover function which will limit discovery to services in that group [UPDATE: this last adds no real value, since one would normally call discover with the fully-qualified name, nw0.discover("TeamA/robot")]

[tjguk]

Implemented via 0caa4aa20a6d4ab667d178a58056aba4ad892295