Simple instance variable caching.

The problem with the last commit is that we ended up making a LOT MORE
requests than we should have, due to making excessive XML objects. Then
inside of the XML object, we referred to the virtual attribute that
makes a Request multiple times. Ack! Now we don't do that any more.

Note that HTTP caching would be the 'right' thing to do here. I don't
want to get into that yet, so we're just using instance variables.

maze.rb

@@ -4,6 +4,7 @@
 
 class Room
   def initialize(uri)
+    @xml ||= {}
     @uri = uri
     @uri = xml.extract_href('start')
   end
@@ -23,7 +24,7 @@ def finished?
   private
 
   def xml
-    XML.new(@uri)
+    @xml[@uri] ||= XML.new(@uri)
   end
 end
 
@@ -33,7 +34,7 @@ def initialize(uri)
   end
 
   def current
-    Request.new(@uri).get
+    @request ||= Request.new(@uri).get
   end
 
   def extract_href(rel)