index of posts of a given category is more logical

there is an attribute for_category of the PostIndexTemplate class
(Boolean, default False) and, if True, there is a non-Null
category attribute (string, default 'other')
content for home and about page
bug in post index by category solved

* link to home doesn't link to blog index anymore, but to home.
* change in class PostIndexTemplate - self.all_posts defined at the
end, after any filter by category (if there is any)