Integration tests for RabbitMQ

[Kralizek]

• RabbitMqBusEngine now uses new BufferSubject to handle the condition race between connection to RabbitMQ and subscription of the handlers
• IBusHost does not expose the SubscribeToCommand and SubscribeToEvent methods anymore
• IContext now expose Message for later uses
• NybusDispatcher now accepts the whole message, not just the correlationId
• Moved Integration test projects under tests/integration
• RabbitMq integration project uses FakeRabbitMQ to fake a connection to a RabbitMQ node
• RabbitMq integration project has a set of test pointing to a real RabbitMQ node, these tests are excluded during the normal CI pipeline

Fixes #24