Create prototype of animation subsystem

[nickiannone]

Implement the base classes and animation system, and get a simple prototype running.

Goals:

• Implement animation system prototype, based on what is described in /docs/animation.md.
• Create a stick figure Mesh.
• Create stock skin assets.
• Create a simple AnimSet consisting of "Idle_RightForward" and "Walking_RightForward" animations, with a couple of frames each.
• Create an AnimatedObject which uses the Mesh and AnimSet.
• Bind some keys to switch back and forth between "Idle_RightForward" and "Walking_RightForward".
• Implement an FPS counter to render to the screen.
• Hook up the animation system to the GameWindow, and start rendering.
• (Optional) Add pause, slowdown, speedup, and reverse controls.

[nickiannone]

Frame tweening complete, untested. Need to work on other aspects of animation pipeline (namely, injecting animation timing, applying Frames to a Mesh, and generating AnimBlocks), integrate into Gosu draw/update model, and build first prototype.

[nickiannone]

Created Mesh, Vertex and Edge classes, and incorporated application of Frame to Mesh, and generation of AnimBlocks. Need to incorporate into Gosu GameWindow, create prototype assets and mesh/animation structures, and build rest of prototype code.