RESTful KT API: Replace "Entry" objects with "User" objects

[gdbelvin]

The KT api currently does not really look like a RESTful API. This is because, unlike most restful APIs, KT is returning a lot of cryptographic "proof" in each response.

This can be improved by getting and posting a single "User" object which may optionally include a "UserCommitment" and "UserProof". The "UserCommitment" would be an obfuscated object that gets put in the Merkel Tree, while the "UserProof" would tie the data in "User" to the "UserCommitment".

[gdbelvin]

API Cleanup Actions:

• [x] Rename Epoch with Revision for consistency with the Trillian Map.
• [x] Rename Domain with Tree for clarity