Make Ebi move, refactor bullet code to make more logic per-entity type.

src/entity.lua

@@ -12,6 +12,10 @@ function Entity:init(img, health, armor)
    self.health = health or 10
    self.armor = armor or 0
 
+   -- movement direction, every update() the entity will move along this vector and return
+   -- collision data to the subclass
+   self.vector = {x=0,y=0}
+
    self:setCollideRect(0, 0, self:getSize())
 
    -- most entities will be enemies, so we configure this mask by default
@@ -29,3 +33,8 @@ function Entity:damage(amount)
       self:remove()
    end
 end
+
+function Entity:update()
+   local collisions = select(3, self:moveWithCollisions(self.x + self.vector.x, self.y + self.vector.y))
+   return collisions
+end