Do not run Nunit 3.0 tests

I have removed the search for nunitlite and now throw a new `NUnit.Core.UnsupportedFrameworkException` if attempting to run a test assembly using NUnit 3.0 or newer. In the console and in the nunit.gui, this gets caught by the general exception block and displays the error. I made it a special exception so that the adapter can handle it differently if Terje chooses.

Fixes #30