Started on the ExtendedDocument class with features moved to mixins.

Properties got added, they define getters, setters and aliases.
They will also be the base of the new validation system.

lib/couchrest/core/document.rb

@@ -1,6 +1,6 @@
 module CouchRest
   class Response < Hash
-    def initialize keys = {}
+    def initialize(keys = {})
       keys.each do |k,v|
         self[k.to_s] = v
       end