A set of IsXEnabled() extension methods and support in Log4Net, NLog and Serilog. EntLib not supported yet. #4
Merge branch 'master' into loglevelcheck

Conflicts:
	src/LibLog.Tests/LogProviders/EntLibLogProviderLoggingTests.cs
Refactor entlib provider for better performance.
Basic idea how we to check whether logging is enabled for a particular kind of message.