Fix uncapitalized order status names in operations view

The orders table of the operations view shows the order status in lower-case,
uncapitalized string. This change fixes that by capitalizing the name of
the order status.
Merge pull request #129 from apoloval/fix/order-actor

Fix a race condition in `OrderActor`
Merge pull request #130 from apoloval/fix/OrderStatusLabels

Fix uncapitalized order status names in operations view
Replace `Connect` message by `Join` in `MessageGateway`

We want the message gateway to join the network rather than connect to it.
The difference between these two terms is that connect implies a network channel
to be established while join means that it will attempt to form part of the
network, establishing as many channels as required and reconnecting them as
they are lost.

This change replaces the term _connect_ by _join_ in message gateway classes.
Merge pull request #131 from apoloval/task/154

Replace `Connect` message by `Join` in `MessageGateway`
Use system event stream to manage event channel subscriptions
Remove event subscription and descubscription from `EventChannelActor`

Since we are using the system event stream, the event listeners can just
subscribe to it instead of requesting anything to the `EventChannelActor`.
Replace `EventProducer` by `EventPublisher`

Instead of sending the event to the `EventChannelActor`, this new trait sends
the event directly to the system event stream. In tests, the event channel probe
is not representing the channel itself but a subscriber.
Remove `EventChannelActor` from the equation

Remove that actor and stop using it from the actor system. The system event
stream is used instead.
Merge pull request #133 from apoloval/task/164

Replace `EventChannelActor` by the system event stream
Make Exchange.Progress additive
Replace Exchange trait by an Exchange case class

Use Exchange.State to model the distinct states and state-dependent
exchange data. Also add some pimps to easily move from one state to the
next one.
Merge both SampleExchanges to avoid duplication
Move SampleExchange to the model module
Simplify OrderTest with SampleExchange
Rename NonStarted to NotStarted
PR feedback
Merge pull request #132 from sortega/task/165_exchange

Replace *Exchange hierarchy by a Exchange case class