datastore: scope Key creation to include NS. fixes #116.
datastore: support overriding namespace.
datastore: support namespace overrides & removal.
datastore: tighten up falsy namespace values.
datastore: prefer `namespace` over `ns` naming convention.
datastore: readme: mention default namespace scoping.
datastore: undefined -> null.
Merge pull request #124 from stephenplusplus/namespaces

datastore: scope Key creation to include NS. fixes #116.