Signatures should be on hashes

[ebuchman]

Signatures actually shouldn't be part of the same pdf document, since once you sign it you can't check that its the same document as before (unless you use eg. pdf-diff, related to #30).

Instead, signatures should probably be on some separate document that just contains the hash of the contract pdf. For electronic signatures this is possible by signing a contract that refers to the hash of the actual contract. For digital signatures, this is of course native.