improved testbed and serialization

[Daniel Murphy]

Hello!

Before beginning the update to 2.2.1, I wanted to release the current additions I have made to the engine.  First, I've improved and modularized the testbed.  While it's still not perfect, the graphics, ui, and run loop are much more separated from the tests + settings, which should make custom implementation much more straightforward and custom tests a lot easier to create.  The new wiki page is here.

Secondly, I've included world serialization with Protocol Buffers, which serializes (and de-serializes) all the physics elements in the given world (so not a complete world-state serialization with contacts, etc).  The framework supports user tagging for each physics entity, and more formats can be implemented if desired.

If you decide to update, let me know if you have any issues, I'm happy to help.

Thanks,

Daniel Murphy