* Cause a parse error if user defined matchers use a mac tag that is
not a normalized MAC address. This prevents users from silently defining
tags which will never match a machine.
* Fixes#25
* Use YAML group config to match machine labels to groups of zero or
more machines sharing a Specification
* Discontinue using machine.json resources representations which are
repetitive, don't support groups, and don't allow label matching
* Previous "default" special case now handled by a Group with an empty
matcher
