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.

test/liquid/tags/standard_tag_test.rb

@@ -4,7 +4,7 @@ class StandardTagTest < Test::Unit::TestCase
   include Liquid
 
   def test_tag
-    tag = Tag.new('tag', [], [])
+    tag = Tag.parse('tag', [], [], {})
     assert_equal 'liquid::tag', tag.name
     assert_equal '', tag.render(Context.new)
   end