Added assets manager singleton.

src/graphics/drawable.lua

@@ -16,7 +16,8 @@ local Drawable = make_class(Asset)
 -- Class methods
 ------------------------------------------------------------------------------
 
-function Drawable:_init()
+function Drawable:_init(file_name)
+    Asset._init(self, file_name)
 end
 
 

src/graphics/sprite.lua

@@ -17,15 +17,15 @@ local Sprite = make_class(Drawable)
 -- Class methods
 ------------------------------------------------------------------------------
 
-function Sprite:_init(sprite_name, x, y)
-    self.sprite_name = string.format('assets/%s', sprite_name)
+function Sprite:_init(file_name, x, y)
+    Drawable._init(self, file_name)
     self.x = (x ~= nil and x) or 0
     self.y = (y ~= nil and y) or 0
 end
 
 
 function Sprite:load()
-    self.sprite = love.graphics.newImage(self.sprite_name)
+    self.sprite = love.graphics.newImage(self.file_name)
 end