Improve C++ getting started instructions

Link to "Test-Driven Development in C++" workshop from C++ Now! 2014.
Break out Linux/Windows/MacOS instructions more explicitly.
Add more Windows specific instructions for using pre-built Boost.
Explain how to keep your build cruft separated from your source.
Link to exercism/xcpp/issues for potential problem spots to solicit
feedback in improving the experience.
Merge pull request #89 from LegalizeAdulthood/cpp-tdd

Improve C++ getting started instructions