Add System.Composition (MEF2)

This adds System.Composition (also known as MEF2 and Microsoft Composition) to CoreFx,  The code was previously available at [mef.codeplex.com](http://mef.codeplex.com/).

I tried to match the existing CoreFx directory structure.  There are 5 different assemblies that make up System.Composition, but there are various test, demo, and sample projects which don't directly correspond to a single product assembly.  So I've created a separate directory under src for each of the product assemblies, while I've put the tests and the rest of the projects under src/System.Composition.