Introduce unit tests which use NUnit. NUnit is now a dependency
so developers need to install it via NuGet.

There aren't many tests yet. Just a few for Core and Basetypes but
hopefully more will be added in the future.

More information can be found in Source/UnitTests/README.md
Make run-unittests.py executable under Linux/OSX. This
required changing the line endings so that it uses unix
line endings.
Patch by Jeroen Ketema

Fix interfacing with CVC4 1.5