Add possibility to customize Exchange creation

[Kralizek]

RabbitMQ options now support the properties CommandExchange and EventExchange.

Here is a sample of the configuration layout

{
    "Nybus": {
        "RabbitMq": {
            "CommandExchange": {
                "IsDurable": true,
                "IsAutoDelete": false,
                "Properties": {
                    "some-property": "a value",
                    "some-other-property": "another value"
                }
            },
            "EventExchange": {
                "IsDurable": false,
                "IsAutoDelete": true,
                "Properties": {
                    "some-property": "a value",
                    "some-other-property": "another value"
                }
            }
        }
    }
}

Fixes #75