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.
It depends on the formatting defined in NLog.config. In the sample it's 

[level:logger] state
exception

so if there's no state it would be 

[level:logger]
exception

I think it makes sense to have a new line before the exception even if there is no state, and I believe [this code](https://github.com/aspnet/Logging/blob/dev/src/Microsoft.Framework.Logging/LoggerExtensions.cs#L16) also has a new line even if there's no state. Does the `CultureInfo` make sure the `\r\n` is rendered correctly on all OSes?