Feature: add 2 kinds of aliens
Refactoring: better way to manage aliens
Refatcoring: creating models and animations folders to manage independent files
Bugfix: right alien type in 1st wave
Add license
Refactoring: move function to tools.js
Refactoring: better way to manage weapons
Bugfix: right alien definitions in waves
Optim: better way to manage offset for each wave rows