refactor(target): Rename target pkg to unit
refactor(unit): Break out Target from unit.go into manager.go
refactor(unit): Rename Target to SystemdManager
refactor(unit): Move all local systemd/filesystem access to SystemdManager
refactor(unit): Break out dbus path serialization and test it
refactor(unit): Solidify contract between Agent and SystemdManager

The Agent should always be passing around Job objects, not
job names.
refactor(unit): SystemdUnits are responsible for fetching their own Payload
Merge pull request #22 from bcwaldon/manager-refactor

Refactoring of local Systemd management