Add query proof

[greatect]

Resolving part of issue #25:

• Use IAVL tree to maintain state and provide merkle proof
• Deal with app height, hash, epoch and put them into ABCI info
• Provide proofs inside ABCI query responses
• Add a test for posting document and querying documents with proof
• Add utilities for generating documents, but still not auto generated during the commit stage
• Remove duplicate signing/verification of document
• Refinement on storage keys
• Naming on error codes