WIP: Alternate model management system.

[hoytak]

This class proposes a new method for managing extensions. It is meant
to replace the bulky and unweildy macro system in use currently.

The new code is contained in src/model_server_v2/.

This is meant to replace the 5K lines of very complicated code surrounding the toolkit_macros and unity server.

Features:

• Far more concise implementation using native C++11.
• Macros are now isolated to class registration and function registration.
• Registration is now performed by a single macro call in header or source file.
• Proper overloading of const methods.
• Registration of methods performed by calls in the constructor; no need for a separate macro section.
• A singleton global model server to replace unity_global (eventually).

Integration plan:

1. Get feedback on this version, iterate.
2. Implement save, load, and other methods needed for full unity integration.
3. Set this as a "fallover" model server -- if a model is not present in unity_global, look for it in this one.
4. Incrementally move models over to this version.
5. Remove unity global when all models are converted.

Review notes:

Begin reviewing it by looking at the examples in demo.cpp. This is a fully working example of what the registration looks like.