Included server methods and minor changes in other modules.

Included:
- 'is_a_valid_message' method to validate the received
message from a client.
- 'send_primes_in_range_to_client' method to find primes
in the given range and send them to the client.

Minor changes in:
- 'test_client.py' to standarize the lists declaration.
- 'constants.py' to standarize the dicts and strings declaration.

Pending:
- 'test_server.py' in order to test the server's methods.

Included