SignigKey: add read and write methods for private key:

cleaner separation of implementations between the software and the lib:

• the client does not need to import libnacl
• save() method already exists:
  • The new method allows its presence in the documentation
  • it could be find reading the example
  • as a beginner it’s not obvious to find it from the code as you have to understand super() and the inheritance schema in libnactl code.

update save_and_load_private_key_file example