More modular context implementation (custom renderers)

Context is a bit hardcoded atm, and it's something I've wanted to change for a while. I would really like to see this refactored so that custom renderers can register their own commands.

For example, currently we handle the GL_AMBIENT_LIGHT command as a part of the switch statement in compositor.

`Context.js`

```
case 'GL_AMBIENT_LIGHT':
if (!this.WebGLRenderer) this.initWebGL();
this.WebGLRenderer.setAmbientLightColor(
path,
commands[++localIterator],
commands[++localIterator],
commands[++localIterator]
);
break;
```

A more modular way to do this would be to have the renderer have a list of its commands to register.

`WebGLRenderer.js`

```
WebGLRenderer.COMMANDS = {
'GL_AMBIENT_LIGHT': 'setAmbientLightColor'
}
```

And context would simply register renderers and all their commands.

`Context.js`

```
this.addRenderer('webgl-default', WebGLRenderer);
```

As far as handling how many arguments are passed in, I would propose that we just pass the path, command array, and local iterator into every handler function. For example:

`WebGLRenderer.js`

```
WebGLRenderer.prototype.setAmbientLightColor = function setAmbientLightColor(path, commands, iterator) {
this.ambientLightColor[0] = commands[++iterator];
this.ambientLightColor[1] = commands[++iterator];
this.ambientLightColor[2] = commands[++iterator];
return iterator;
};
```

I think this would make both adding new renderers and adding new commands easier.

This would also let us avoid having this everywhere in Context:

`Context.js`

```
if (this.webGLRenderer) {
...
}
```

P.S. I know https://github.com/Famous/engine/pull/281 is changing the switch statement but it shouldn't affect this implementation all that much.