Make enum_ work with integer types again.

Also/as a side effect simplify error checking while also detecting loss of
precision (e.g. float -> int).

This broke compilation of test_scope.cpp
Add tests to build (incl. CTest support).

You can set the LUABIND_SKIP_TESTS CMake cache variable to a true value to skip
them (there are a lot of test projects and the folder property does not seem to
work at least at VS Express).