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
Merge pull request #552 from bcwaldon/API-unit-crud

API, pt IV: CRUDdy Units