Include Catch as a git submodule.

Bug: #40
Include RtMidi as a git submodule.

Currently, we are still using version 2.0.1 - at some point we should
test out 2.1.0.

Bug: #40
Include RapidJSON as a git submodule.

This includes a few updates for recent API changes.

Bug: #40
Include pugixml as a git submodule, and update to v1.4.

Bug: #40
Add a note about initializing submodules when cloning the repo.
Merge pull request #41 from cameronwhite/submodules

Fix #40 - Use submodules for third-party libraries.