class PostTemplate and SimpleTemplate inheritance; post.is_latest

* SimpleTemplate and PostTemplate inherit from
PageContentTemplate; PostIndexTemplate inherits from PageTemplate
(it's instantiated from the posts, but not from a txt or html
file!)

* the variable is_latest for PostTemplate has been commented out
(and post_template consequently modified). The problem should be
solved in another way (list of posts in decreasing order by date)
check of category for index for category on url, not string

otherwise, conflict if two strings lead to the same url
(dictionaries have an unique set of keys!)
minor changes in template