Add acceleration (setAcc and .acc)

Added acceleration support for kinematics.

Example code:
```
var Example = function(io) {
	var dir = 1;
	
	var box = io.addObj(new iio.ioRect(500, 25, 10, 10)
		.setFillStyle('blue')
		.enableKinematics()
		.setAcc(0.1, 0)
		.setBound('right', 550, function(box) {
			if(dir == 1) {
				dir = -1;
				box.setAcc(-0.1, 0);
			}
			return true;
		})
		.setBound('left', 500, function(box) {
			if(dir == -1) {
				dir = 1;
				box.setAcc(0.1, 0);
			}
			return true;
		}));
	
	io.setFramerate(60);
};
iio.start(Example);
```