First commit toward a version of the adapter using NUnit 3.0

WARNING: This PR should NOT actually be merged with Master.

I'm setting it up so we can easily view changes from the NUnit 2.6.3 version of the adapter to the NUnit 3.0 version. It gives us a single-purpose discussion forum and the ability to make line-by-line comments.

This first commit has simply the 2.6.3 framework with 3.0 alpha release, using a NuGet package reference. I fixed the tests to compile but I would not expect them to run reliably yet under the adapter, which doesn't understand how to run 3.0 tests and will try to use reflection to execute them.