Added generator for creating "awaitable" tasks

This addresses issue #309 

`Fixture` is now able to resolve awaitable generic/non-generic tasks.

```csharp
new Fixture().Create<Task>().Wait();
Console.WriteLine("Task execution completed..");

new Fixture().Create<Task<string>>().Wait();
Console.WriteLine("Generic task execution completed..");
            
Console.ReadKey();
```

In the case of generic tasks, the `Result` is resolved by the fixture.