Added new Claim event, signed event data

In this version UserDataKey events were renamed to Discovery events, and a new event Claim was added. The Discovery events are used to publish hashes to the event log. It allows you to order data that you discovered, but not attempt to assign any ownership to it. The new event Claim exists to claim ownership over that same data. For example, one use of the chain would be to allow graphics designers to hash their images, sign them with a private key, and then publish Claim events to effectively claim copyright on those images. Those observing the log would not be able to see the image, but should they get a copy, they could track down the copyright holder via the Claim's public key.