function getAngle(p1, p2, props) {
	    if (!props) {
	        props = PROPS_XY;
	    }
	    var x = p2[props[0]] - p1[props[0]],
	        y = p2[props[1]] - p1[props[1]];
	    return Math.atan2(y, x) * 180 / Math.PI;
	}
calculate the angle between two coordinates
@param {Object} p1
@param {Object} p2
@param {Array} [props] containing x and y keys
@return {Number} angle