Remove addins

This is the first stage of removal of NUnit Addins from the framework. I'd like to get this merged before pushing any more commits to it, since I think it will become more difficult to review.

As it the code now stands, there is no longer code in the framework to handle addins. Many of the previous internal addin classes are still present, but they are no longer treated as addins.

Once this is merged, the next stage will be to move code into attributes as described in the spec.