Feature loggingprovider

Adding concept of ILoggingProvider which can be added to ILoggerFactory

Creates a single default ILoggerFactory that does not need to be replaced to add 3rd party logging tech

Makes ILogging* interfaces AssemblyNeutral, and moves them into a standalone package
I was wondering about that - which is why I copied the portable target definition in the project.json also...

The assumption there is that if you're not using vnext from EF, and you're not our host, in you'll end up with the PCL version of Microsoft.Framework.Logging.Interfaces that has the interfaces sitting there as plain old types...

I'm hoping that will keep EF working until we sort out the "now ANI works all over the place" questions