Ruleset and randomizer refactoring (Read comments)

You can now specify an arbitrary number of pieces for a ruleset.
The randomizers will adjust accordingly.
Expect a pento ruleset in the modpack soon!
Also, gamemode skin selection has been refactored.

tetris/randomizers/bag.lua

@@ -0,0 +1,23 @@
+local Randomizer = require 'tetris.randomizers.randomizer'
+
+local BagRandomizer = Randomizer:extend()
+
+function BagRandomizer:new(pieces)
+    self.bag = {}
+    self.pieces = pieces
+    for i = 1, self.pieces do
+        table.insert(self.bag, i)
+    end
+end
+
+function BagRandomizer:generatePiece()
+	if next(self.bag) == nil then
+		for i = 1, self.pieces do
+            table.insert(self.bag, i)
+        end
+	end
+	local x = math.random(table.getn(self.bag))
+	return table.remove(self.bag, x)
+end
+
+return BagRandomizer