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