test(job): Expand job pkg tests
refactor(job): Separate systemd logic into new file in job pkg
refactor(registry): Use an (un)marshal helper for JSON (de)serialization
refactor(registry): Store JobPayload objects in etcd as JSON
refactor(registry): Expand json serialization error reporting
Merge pull request #18 from coreos/etcd-registry-json

Refactor job pkg, adding tests; serialize JobPayload as JSON into etcd