Add raw tag. Fixes #37.

Thanks to phaer: https://gist.github.com/1020852

YO DAWG, WE HEARD YOU LIKE LIQUID

SO WE PUT A RAW TAG IN YOUR LIQUID
SO YOU CAN HAVE LIQUID IN YOUR LIQUID

lib/liquid/tags/raw.rb

@@ -0,0 +1,21 @@
+module Liquid
+  class Raw < Block
+    def parse(tokens)
+      @nodelist ||= []
+      @nodelist.clear
+
+      while token = tokens.shift
+        if token =~ FullToken
+          if block_delimiter == $1
+            end_tag
+            return
+          end
+        end
+        @nodelist << token if not token.empty?
+      end
+    end
+  end
+
+  Template.register_tag('raw', Raw)
+end
+