mod_pep_atom_categories: Add a module to index categories in Atom entries A new node is created for each category, named category-<the category>, which contains a list of all node/item tuples having said category as a tag.

This module blocks configured queries that change server state.

E.g. to make vCard storage read-only:

``` lua
readonly_stores = {
	vcard = { "vcard-temp", "vCard" };
}
```