Issue #3: Refactoring ILogger

Moved extension method `IsEnabled` to the `ILogger` interface, renamed `WriteCore` to `Write` and changed the return type to void. This will break other repositories that use the ILogger interface, which I will update once I get feedback on the changes here.
`CultureInfo` doesn't affect newlines at all. It's more about the underlying OS (Windows, OS X, *nix, etc.).

I think I see your point about the newlines. So you always want the exception on its own line no matter what? I can live with that.