refactor(payload): Simplify JobPayload

* JobPayload is now just a Name and Unit
* Break the SystemdUnitFile data object into its own package
* Move code out of unit package to systemd package that interacts with systemd
feat(job): Implement new Job flow

* Jobs are now submitted, started, stopped, then destroyed
* Lots of refactoring internally to simplify Registry calls
* Drop unused events EventJobStatePublished, EventJobStateExpired
* Rename EventJobCancelled to EventJobStopped
Merge pull request #66 from bcwaldon/corectl-verbage

Implement new Job flow
Merge pull request #65 from robszumski/master

feat(docs): add short service discovery doc
doc(corectl): Expand docs on corectl
Merge pull request #67 from bcwaldon/docs

doc(corectl): Expand docs on corectl
doc(corectl): Update command usage
fix(corectl): drop accidental log statement
doc(corectl): Complete documentation of corectl commands
Merge pull request #68 from bcwaldon/docs

moar corectl docs
refactor(ssh): Move ssh code to new pkg; prettify
feat(corectl): Implement ssh command
Merge pull request #69 from bcwaldon/corectl-ssh

corectl ssh
feat(docs): add diagrams