Unresolved symbol - Clara::UnpositionalTag

The first commit adds a new test executable which doesn't use Catch's own `main()` function.  It fails to link using MSVC 2012; I believe this is the same problem as https://github.com/philsquared/Catch/issues/252.

The second commit fixes the issue for me - I'm not sure if it's the best way to fix the problem though?

I've also only added the new executable to the CMakeLists.txt, not to any other makefiles or project files.  I strongly feel that these shouldn't exist if you're providing a CMake option.

The whole project would be easier to work with too if you could merge in https://github.com/philsquared/Catch/pull/215 - all the files show up in the IDE that way.  That pull request is probably out of date by now.  If you want, I can update it if you plan to merge it.