Update src/core/Vector3.js

Would be great if the function to get angle to anonther vector could be implemented in this Vector3 class.
Hope you guys like the idea.

src/core/Vector3.js

@@ -249,6 +249,12 @@ THREE.Vector3.prototype = {
 
 	},
 
+	angleTo: function ( v ) {
+		//angle between this and v:
+		//this 'dot' v = |this|*|v|*cos(alpha), where alpha is the angle
+		return Math.acos( (this.x*v.x+this.y*v.y+this.z*v.z) / (Math.sqrt(Math.pow(this.x,2)+Math.pow(this.y,2)+Math.pow(this.z,2))) / (Math.sqrt(Math.pow(v.x,2)+Math.pow(v.y,2)+Math.pow(v.z,2))) );	
+	};
+
 	distanceTo: function ( v ) {
 
 		return Math.sqrt( this.distanceToSquared( v ) );