Deprecate md5 document hash and introduce more secure algorithm(s)

[mrosendin]

Reasons to replace MD5:

• It is insecure - prone to collision hashes.
• not a modern algorithm, keccak256, sha256, or ripemd160 checksum hashes are preferred in this order

Why Keccak256 is preferred:

• keccak256 is slightly faster than sha256 and is natively supported on Ethereum
• is future-proof, consider it the next generation of sha256
• Enables a more tight integration of OCF with blockchain implementations

Why sha256 might be the best default candidate for now:

• Modern browsers have native sha256 support whereas keccack256 requires 3rd party libraries
• widely used

It might be beneficial to have multiple hashes in the object.