Implement the ability to store uploads on a per-page basis

Storing all uploaded files in a single directory kinda sucks when you've got
a largish wiki, or the possibility of filename collisions.  With this patch,
though, you can set `:per_page_uploads => true` in your wiki settings and
have the file uploaded to a directory named for the page you were on when
you clicked 'Upload'.

lib/gollum/views/page.rb

@@ -42,6 +42,10 @@ module Precious
       def allow_uploads
         @allow_uploads
       end
+      
+      def upload_dest
+        @upload_dest
+      end
 
       def has_header
         @header = (@page.header || false) if @header.nil?