fix(test): Update test runner
refactor(job): Move JobPayload.Type to Job.Type

The Type field is much more relevant to the Job rather than the
payload that job contains. Remove the JobPayload.Type attribute
and add a Type attribute to the Job struct.
refactor(job/target): JobState objects are now produced by SystemdUnits

* Create SystemdUnit interface to abstract SystemdSocket and SystemdService
* JobState now knows about a list of (string) sockets
Merge pull request #21 from bcwaldon/port-tracking

Refactoring for the ability to expose sockets out to clients