Fixed typo
Add a new alien sprite node
Added a physics body to the alien
The alien's physic body is dynamic. It is moved from the physic of the game scene
Added category for both alien and photon torped
Added the categoryBitMask to AlienCategory
ContactTestBitMask is equal to photonTorpedoCategory
Set the collisionBitMask
Random position for the alien
Fixed issue with the alien position
Adding the alien to the scene
Added variable for the duration of the animation
Added actions to the alien
updateWithTimeSinceLastUpdate: Checking when to add a new alien
Update the update function
Fixed position
Fixing an exception caused from a wrong duration