Nunitlite runner in separate assembly

**For Discussion, don't merge yet**

This PR is intended to help with the discussion on the dev list about supporting self-executing tests.

I have removed the Runner directory entirely from the framework. Since it was only present in nunitlite, this eliminates one more key difference in the two frameworks.

The nunitlite tests are still runnable but now reference both framework and runner assemblies. If desired, we could also make the runners executable on some platforms, allowing them to be used in a more traditional way.
At some point, it would be nice to go through the projects and clean up a bit, switching things like the OutputPath to use the $(SolutionDir). I think that might be broken with MSBuild, but expect we could define it.