SCHEMA-688. Create EntityIds from JSON strings/nodes.

KijiREST introduced a different way to parse entityIds from JSON nodes or JSON
strings which made it easier to specify EntityId strings in the URL. Since a few
other components can use this, we moved this logic into KijiSchema so other
components can depend and use this.

Reviwer: shashir, ajprax
Review: https://review.kiji.org/r/1747/