Added support for checking for *any* file result.

As per our earlier discussion (#3) 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 any breaking changes.

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.