[enh] #95 add optional signing_key argument in Document sub-classes

duniterpy/documents/block.py

duniterpy/documents/certification.py

duniterpy/documents/identity.py

duniterpy/documents/membership.py

duniterpy/documents/peer.py

duniterpy/documents/revocation.py

duniterpy/documents/transaction.py

examples/save_revoke_document.py

examples/send_certification.py

examples/send_identity.py

examples/send_membership.py

examples/send_transaction.py