Merge pull request #301 from bcwaldon/update-build-release

fix(build-release): Drop multi-OS support from build-release
fix(fleetctl): make -v work for displaying versions
also do shortcut for -v
make gofmt happy
Merge pull request #306 from jonboulle/master

fix(fleetctl): make -v work for displaying versions
refactor(registry): Return boot ID from GetJobTarget

No callers of Registry.GetJobTarget need a full MachineState object. Rather
than create a dummy object, GetJobTarget can simply return the boot ID.
Merge pull request #311 from bcwaldon/refactor-GetJobTarget

refactor(registry): Return boot ID from GetJobTarget
refactor(requirements): Move requirements-parsing to JobPayload

SystemdUnitFile.Requirements is actually fleet business-logic, so it
belongs with in JobPayload.
refactor(payload): Move fleet option constants to job pkg

The meaning of the X-Fleet parameters belongs to Payload, so the
constants should be defined in the job package.
doc(job): improve comment
test(payload): clarify failure message in test
Merge pull request #310 from bcwaldon/relocate-requirements

Move payload-related business logic into job package
refactor(payload): clean up payload type-detection
feat(payload): support timer units

* Allow users to submit systemd timer units
* Add a default peer to systemd timer units similar to the
  handling of socket units