#95, #172: BBC: Arguments changes on 'Documents' constructors: Remove 'signature', introduce 'signing_key', make 'currency' and 'version' optionals

Break backward compatibilities!

• Remove useless signature argument from Documents.__init__()
  • Rename Identity(ts: BlockUID) by Identity(timestamp: BlockUID)
• Refactor Document.signatures (List) as Document.signature (str)
  • Transaction can have multiple signatures, with Transaction.signatures attribute and Transaction.multi_sign([keys]) method.
  • Move signatures verifications in the Documents classes
• Add optional signing_key argument in Document sub-classes
• Refactor subclass version argument as optional with default=subclass.VERSION
• Set time argument optional in SimpleTransaction
• Refactor Documents currency arguments as optional with default=constants.CURRENCY_CODENAME_G1

Close #95 (closed), #172 (closed).