Added C++11 unit test for Stack-Machine solution

Also added a simple Makefile to simplify running unit tests for different languages.

<del>Including googletest ends up adding roughly ~1MB to the repository size, and frankly, an absurd amount of additions. Instead, should I add it as a submodule, and have the Makefile pull the files if actually needed?</del>
Changed it to a submodule that gets pulled if necessary.