function Knob(o)
{
	this.enabled = true;

	this.value = 0;
	this.delta = 0.01;

	this.steps = 0; //0 = continuous
	this.min_value = 0;
	this.max_value = 1;
	this.min_angle = -120;
	this.max_angle = 120;
	this.axis = vec3.fromValues(0,0,1);

	this._dragging = false;

	if(o)
		this.configure(o);
}
Knob allows to rotate a mesh like a knob (rotate when dragging)
@class Knob
@constructor
@param {String} object to configure from