I lost over a day trying to figure out why I couldn't set a working directory...

I lost over a day trying to figure out why I couldn't set a working directory for tests. It turns out, you can't (at least for Visual Studio projects) if you used gtest_add_tests, but it works just fine using the more up to date gtest_discover_tests. This PR is written to facilitate two very common needs of testers: access to a working directory and the ability to include libraries (since, tests are often written to tests libraries.)

Changes: Replace gtest_add_tests, which is outdated and causes problems (especially in Windows & visual studio) with gtest_discover_tests. Modified macros to show how working directories are set Add example that sets working directory and uses custom libraries. Add include(GoogleTest), which seems to be necessary on some platforms