Rework send membership and identity documents examples

• [enh] Improve send_identity.py example
  • Pass the key to get_identity_document()
  • Rename the file to send_identity for consistency with the other files
• [enh] Improve send_membership.py example
  • Remove the useless Identity document generation
  • Pass the key to get_membership_document()
  • Get the uid and the timestamp from the identity get from /wot/lookup
• [mod] Improve bma imports in send_*.py examples