[FIX] website_blog: Fixed the next post loop issue.

When all blog posts are been visited by the user then clicking on "Read Next" should start looping into those visited blogs again, but currently, it stuck looping into the same blog post(or 2 blog posts), video ref: https://docs.google.com/a/openerp.com/file/d/0BxqHe5rtIxY0UTl2Rmlkb2JkTkk/edit

Now, in the current code, search on "blog.post" object performed twice and each has a problem, first search was performed ordering by "ranking", a function field, which is not supported and second search was performed on "id" of "blog.blog" object. This branch improves this.

With this fix, when client visits the blog post first time(that is, 'visited_blogs' cookies is not set) it will display the most visited blog posts first and then once all blog posts are been visited it will restart listing the blog posts referring the 'visited_blogs' cookies(that is, in the same sequence as it was displayed first time to client).

Kindly review it and share your feedback on it.
Thanks.