Multiple database support for ExtendedDocument.

New optional parameters are available to select the database:

Mixins::DocumentQueries
  * get <id>, <db>
  * all :database => <db>
  * first :database => <db>

Mixins::Views
  * view <name>, :database => <db>
  * all_design_doc_versions <db>
  * cleanup_design_docs! <db>

Mixins::DesignDoc
  * refresh_design_doc now only updates the design_doc _id and removes _rev
  * call save_design_doc to save and update the design_doc
  * call save_design_doc_on <db> to save the design doc on a given
    database without modifying the model's design_doc object

Design (core/design.rb)
  * new method view_on <db>, ...

Bug fixes:
  * design_doc_slug in mixins/design_doc.rb was using an empty document
    to calculate the slug each time
  * method_missing in core/extended_document.rb now passes a block through

lib/couchrest/more/extended_document.rb

@@ -77,10 +77,10 @@ module CouchRest
     end
     
     # Temp solution to make the view_by methods available
-    def self.method_missing(m, *args)
+    def self.method_missing(m, *args, &block)
       if has_view?(m)
         query = args.shift || {}
-        view(m, query, *args)
+        view(m, query, *args, &block)
       else
         super
       end