Basic collision framework

I've added a basic collision framwork to check if two objects collide.
The collision framework checks the collisions with a static width and
height, I mean that the rectangle size in wihch the framework works
isn't updated so different image frames size aren't used
Ship can shoot.

The ship can shoot now pressing SPACE key.
Improved shots system

I've changed the way to shoot. Shot key is SPACE but now you have to
release the key to shoot again.
I have also modified the position where the bullet appears. It now
appears at right side of the spaceship and more or less centered at the
tip.