Added support for checking for *any* file result.

As per our earlier discussion I added a method called `ShouldRenderAnyFile` that will effectively assert that a controller result inherits from `FileResult`. I also arranged the file tests across the solution into regions to improve readability. This pull request does not introduce a breaking breaking change.

Just one question. One of the tests looks like this: 

```c#
[Test]
public void Check_for_any_file_result() {
    _controller.WithCallTo(c => c.EmptyFile()).ShouldRenderAnyFile();
}
```
I was wondering if it would be correct to add some additional assertions:
```c#
[Test]
public void Check_for_any_file_result() {
    _controller.WithCallTo(c => c.EmptyFile()).ShouldRenderAnyFile();
    _controller.WithCallTo(c => c.EmptyFilePath()).ShouldRenderAnyFile();
    _controller.WithCallTo(c => c.EmptyStream()).ShouldRenderAnyFile();
}
```

Thanks Rob.