Many changes all around. Started main menu.

Added Drawable, Sprite and SoundEffect classes.
Added comments to Fader class.

src/states.lua

@@ -2,7 +2,8 @@
 -- Imports
 ------------------------------------------------------------------------------
 
-local Intro = require 'src.gstates.intro'
+local Intro    = require 'src.gstates.intro'
+local MainMenu = require 'src.gstates.menu'
 
 
 ------------------------------------------------------------------------------
@@ -12,6 +13,7 @@ local Intro = require 'src.gstates.intro'
 -- Table of all valid game states.
 local states = {
     Intro('intro', 1),
+    MainMenu('menu', 2),
 }