Feature: add 2 kinds of aliens

Refactoring: better way to manage aliens
Refatcoring: creating models and animations folders to manage independent files

js/spaceinvaders-utils.js

@@ -0,0 +1,12 @@
+function getAliensMidHeight() {
+	var higherAlien = Math.max.apply( null, 
+		$(".alien").map(function() {
+			return $(this).y();
+		}).get() ),
+		lowerAlien = Math.min.apply( null, 
+		$(".alien").map(function() {
+			return $(this).y();
+		}).get() );
+		
+	return (higherAlien + lowerAlien) / 2;
+}