Adding two new constants:

- DEFAULT_TIMEOUT
  The amount of time our client will wait for new tasks before exiting.

- DEFAULT_RETRIES
  The amount of retries our client will attempt.
Adding two new CLI arguments to the `tm-run` command.

The two new options are:

--timeout - This specifies the amount of time the client should wait for new
  tasks from the server.

--retries - This specifies the amount of retries the client should attempt
  before quitting.
Adding --timeout and --retries arguments to tm-slave command.
Adding --timeout and --retries options to tm-spawn command.