fix(registry): Don't blow up if target key not found
refactor(API): Move list out of ServeHTTP
registry: be safer about etcdError type assertions
chore: propagate registry initialization errors in fleetctl
feat(API): Add path detection code
feat(API): Add schema for Unit deletion
feat(API): Implement Unit deletion
feat(API): Add schema for fetching a Unit
feat(API): Implement ability to fetch a Unit
feat(API): Implement dumb GetMachineState
feat(API): Add Unit mutability schema
feat(API): Implement Unit mutation
Merge pull request #552 from bcwaldon/API-unit-crud

API, pt IV: CRUDdy Units
fix(functional): Fix call to unit.NewUnit
Merge pull request #557 from bcwaldon/fix-functional

Fix call to unit.NewUnit in functional tests
Merge pull request #553 from jonboulle/553

fix panic during type assertion in storeOrGetUnit
Merge pull request #556 from jonboulle/registry

chore: propagate registry initialization errors in fleetctl
refactor(registry): s/GetActiveMachines/Machines/
fix(agent): Log registry error in agent
refactor(registry): Remove GetMachineState method