refactor(machine): Separate HasMetadata from Machine
refactor(machine): Drop return value from Machine.RefreshState
refactor(machine): Separate Machine iface from CoreOSMachine impl
refactor(machine): Make refresh an impl detail of CoreOSMachine
fix(machine): Protect CoreOSMachine with mutex
Merge pull request #470 from bcwaldon/machine-interface

Refactor machine pkg to use interface