refactor(state): Rename JobState to PayloadState
refactor(job): Rename Job.State to Job.PayloadState
refactor(registry): Use alternate model for Job marshalling
refactor(state): Rely on Job for PayloadState

PayloadState belongs to a Job, so without a Job a PayloadState object
is useless. Rather than allow clients of the Registry to ask for
PayloadState specifically, force them to get it through GetJob.
Merge pull request #321 from bcwaldon/payload-state

Refactor JobState into PayloadState, owned by a Job