kiji-rest: REST-66. Allow native json formatted entity ids.

Introduces KijiRestEntityId and removes EntityIdComponents.
KijiRestEntityId encapsulates two representations of row keys: using a
string of encoded bytes; or using a json array to specify the
components.

review: https://review.kiji.org/r/1296
issue: https://jira.kiji.org/REST-66
commit: https://github.com/kijiproject/kiji-rest/commit/7d8cfd03e6b3955423f3514b53dabc9e8211c065