Support Derby 0.5.x by storing i18n model information underneath _page

Derby 0.5.x does not allow data to be stored at a root private path, i.e. model.set("_i18n", ...). Instead suggests storing data underneath _page. See the following blog post.

http://blog.derbyjs.com/2013/06/04/migrating-from-derby-0-dot-3-to-0-dot-5/

This commit fixes "Error: set must be performed under a collection and document id. Invalid path: _i18n" fatal error when derby-i18n is used with Derby 0.5.x. However, it MAY BREAK EXISTING SOFTWARE which expects to find i18n information at previously used model path!

This commit builds upon the earlier pull requests by vmakhaev.