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/templates/layout.mustache

@@ -16,6 +16,7 @@
       var baseUrl = '{{base_url}}';
       {{#page}}
       var pageFullPath = '{{url_path_display}}';
+      var uploadDest   = '{{upload_dest}}';
       {{/page}}
   </script>
   <script type="text/javascript" src="{{base_url}}/javascript/jquery-1.7.2.min.js"></script>