[REF] document_page: refactored the module, towards using it in website_blog

[REF] document_page: refactored the module, towards using it in website_blog
Main modifications:
- category is now a blog.catory object, instead of a document.page with type='category'
- added blog.tag model, to be able to tag posts
- cleaned document.page, now blog.post model, removing type and unnecessary features
- added chatter on blog.post, blog.category
- added subtypes to enable direct follow of posts when following a category
- updated document.page.history (now blog.post.history)
- commented menu creation, because this does not seem to be necessary anymore considering the new website
- updated wizard.document.page.history.show_diff, now blog.post.history.show_diff
- updated all views accordingly, as well as demo data
- removed the use of document_user group, simplified rules on blog.post (not done for new models)
- misc cleaning

Next step: merge with website_mail

bzr revid: tde@openerp.com-20130920111221-qy3x5eoyih26axn8