implementation of extension methods to configure databus

Implementation of Fluent API (`ConfigureAzureDataBus`) to provide code-based configuration alternative to config section (`AzureDataBusConfig`).
Both utilize `AzureDataBusGuard` to perform validation.

Please review @yvesgoeleven @johnsimons @SimonCropp 
`AzureDataBusGuard` is also used by [`AzureDataBusConfig.cs`](https://github.com/Particular/NServiceBus.Azure/blob/100c647575895f7e8d6e294fb185e259ec4aab17/src/NServiceBus.Azure/Config/AzureDataBusConfig.cs#L11) via `CallbackValidator` which has to have a callback with object as parameter.