Add NUnitLite dependency.
Add existing IronPython tests.
Add NUnitLite-based runner.
Add console test runner and fix test case loaders.
Factor out test execution & add isolation levels.

Isolation levels allow tests to be run with differing isolation
depending on what each one needs. By default tests are run in a shared
engine with unique scopes.
Break apart CPython test cases so they can be run separately.

Add targets to make.cmd; should probably go to Build.xml at some point.
Fix compilation for .NET 3.5.
Merge branch 'test-runner'
Add a make.cmd command for custom test runs.