Removed header fields from msg.Message struct

All the header fields which were required by the reliable messaging
protocol have been removed. The reliable messaging protocol sends and
receives RMessage structs which contain the header fields plus a Message
struct. The application only ever sees msg.Message structs and so any
message protocol which requires additional data must strip this data off
in the listener and add it back on in the responder. The reliable
messaging protocol does exactly this.

This change was made to allow for multiple messaging protocols to be
implemented without having to constantly add fields to the msg.Message
struct.

The msgutil package has been removed. The set and ticker implementations
have been moved to the coordinator/ package and are now private.
guidutil.go has been moved into the msg/ package.
Added tests for reliable responder

Two tests added to ensure that APP and OUT-ACk messages are written out
to the network appropriately.

The responder now panics if it receives a message from the listener
which isn't an ACK.
Set renamed to rmsgSet and methods made private
RMessage construction in tests now consistent

Previously in treliable_test.go we were constructing RMessage objects in
a different way in each test. Now each construction provides the same
fields in the same order.
Added CopyTo(*msg.Message) method to OrderNode

OrderNodes can now copy themselves into a Message object. This has a
pleasing symmetry because they were already able to copy a Message's
fields into themselves, via CopyFrom(*msg.Message). This replaces the
writeFrom(...) function which existed in matcher.go.
reliableResponder logs with single doLog() method.

This removes the duplicated if statements logging messages received from
the listener and the app.
The scrappy bin/svr now demos basic trading

When you open the page a websocket connection is made. This websocket
connection is attached to a fresh user who gets $100 to trade with. To
see any trades you have to look in the javascript console.

Quite fun to get to here.