send heartbeats from the clients
Refactor Clock class.

* Rename sleep_time() to time_left().
* New method time_passed().
* Added unit tests for Clock class.
New client functions in the network library.

* cli_socket() - creates a socket.
* cli_connect_req() - request connection to the server.
* cli_recv() - receive a message asynchronously.
initial implementation of network manager