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.
For bonus points, you could add `//Arrange`, `//Act` and `//Assert` to your tests (See https://github.com/aspnet/Logging/blob/dev/test/Microsoft.Framework.Logging.Test/LoggerFactoryExtensionsTest.cs#L17). It gives the test some structure making it easier to read.