[enh] #95 refactor Document.signatures (List) as Document.signature (str)...

[enh] #95 refactor Document.signatures (List) as Document.signature (str) (break backward compatibility)

Transaction can have multiple signatures, with Transaction.signatures attribute and Transaction.multi_sign([keys]) method.

verifying_key.py is refactored heavily to avoid circular reference:
* method VerifyingKey.verify_document() removed, use Document.check_signature(pubkey) or Transaction.check_signatures(pubkeys) instead
* method VerifyingKey.verify_ws2p_head() is removed, use Document.check_signature(pubkey) instead.
* Add VerifyingKey.check_signature() method