feat(API): Add ability to list units to schema
feat(API): Implement list units
Merge pull request #549 from bcwaldon/API-units

API, pt III: List Units
fix(registry): Don't blow up if target key not found
refactor(API): Move list out of ServeHTTP
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