Adding ability to get a list of named configurations

[jeking04]

This pull request allows you to get a list of known configurations via:

named_configs = robot.configurations.get_configuration_list()

[psigen]

It seems to me that configurations and tsrs sound like they should be list-like or dict-like, and it is really counter-intuitive for them to have completely custom APIs. Having a call like robot.configurations.get_configuration_list() really seems awkward and not what I would expect to have to do.

But I don't really use this API much, so I'll leave those decisions to you guys.

[mkoval]

@psigen I posted a justification for the ConfigurationLibrary class in personalrobotics/herbpy#81. As always, I'm welcome to hear a compelling counter-argument. 😄