Refactor to create tags with a parse class method instead of new.

By moving parse out of the initializer, we can call super at the start of
the initializers for subclasses, and avoid the nasty allocate hack.

lib/liquid/document.rb

@@ -1,9 +1,8 @@
 module Liquid
   class Document < Block
-    # we don't need markup to open this block
-    def initialize(tokens, options = {})
-      @options = options
-      parse(tokens)
+    def self.parse(tokens, options={})
+      # we don't need markup to open this block
+      super(nil, nil, tokens, options)
     end
 
     # There isn't a real delimiter