Implement support for heartbeats

[achilleasa]

To enable heartbeats, the client must create a new TuningSettings object
with a >= 1sec Duration value for the heartbeatPeriod field. The
TuningSettings must then be passed to the client's constructor via the
ConnectionSettings object.

During the initial connection handshake, the client will calculate the
effective heartbeat period (in seconds) by taking the minimum of the
heartbeat period values suggested by the client and the server.

Once a heartbeat interval has been negotiated between the client and the
server, the client will start sending heartbeat messages to the server
(approximately) twice within each heartbeat period. In addition, the
client also monitors incoming messages from the server. If no message
(heartbeat or not) has been received by the server within the heartbeat
period, the client will raise a HeartbeatFailedException.

The following example illustrates how heartbeats can be enabled when creating a new client:

Client client = Client(
  settings: ConnectionSettings(
    tuningSettings: TuningSettings(
      heartbeatPeriod: const Duration(seconds: 60),
    ),
  ),
);

Fixes #71
Fixes #21

[coveralls]

Coverage increased (+0.5%) to 93.552% when pulling cf0a8c8 on implement-heartbeats into 40f3fc1 on master.