Prevents FatalErrorException being captured twice

See #31

When the stack of handlers is processed (in reverse order) a `FatalErrorException` is passed onto the **second** closure (top of stack) and it will be run as the type hint on `$exception` is `FatalErrorException`. As it returns no response/output, the stack processing continues and it's also run on the **first** closure. As `FatalErrorException` is a sub-type `Exception` it also gets run on that as well.

I will link a pull request for the fix that I have made. I have tested it locally. Let me know if you need any clarification on the above.
@GrahamCampbell I follow PSR2 wherever possible. I only go over the 80 character soft limit for special cases that cannot be split (like file paths). I find it makes it harder to read if your editor window is an 80 character width and you have to horizontal scroll to see the full logic of the *if* statement.

If it troubles you, I can always make the namespace a class constant (as that's what it is!).