Add SetupAsync and StepAsync methods to Report<TOwner>

[YevgeniyShunevych]

In addition to Setup and Step methods of Report<TOwner> new async method versions are added:

public Task SetupAsync(string message, Func<TOwner, Task> function);

public Task<TResult> SetupAsync<TResult>(string message, Func<TOwner, Task<TResult>> function);

public Task StepAsync(string message, Func<TOwner, Task> function);

public Task<TResult> StepAsync<TResult>(string message, Func<TOwner, Task<TResult>> function);