Added first tests for the client package

A pair of very basic tests.

A test showing that the trader sends a specific disconnection message
when the orders channel is closed and then closes the responses channel.

A test showing that the trader sends a specific disconnection message
when a new connection message is received and that the trader then sends
a connection message to the new connection.

The utility method which starts the trader running also creates an
initial connection and tests that a specific connection message has been
sent.

In order to write these tests we had to add specific comments which are
sent for each connection and disconnection event.
Added unit tests for balanceManager

Plus these small changes:

balanceManager now takes the initial balance as an argument.

Added makeResponse method to trader. This allows tests to acquire a
snapshot of the state of a trader without having to listen on the the
responses channel and unmarshal the []byte.