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

src/enemy/ebi.lua

@@ -13,4 +13,18 @@ function Ebi:init()
    Ebi.super.init(self, img, 5)
 
    self:setCollidesWithGroupsMask(0x3)
+   local dir = 1
+   if math.random(2) == 1 then
+      dir = -1
+   end
+   self.vector = {x=0, y=dir}
+end
+
+function Ebi:update()
+   local collisions = Ebi.super.update(self)
+   for i=1, #collisions, 1 do
+      if collisions[i].other:getGroupMask() == 0x1 then
+         self.vector.y *= -1
+      end
+   end
 end