Feature "Categories" complete

* Document plans for categories
* Update blah.otl

* Tweak templates to accomodate category
* Reword post header to better accomodate category
* Increase readability of head.tmpl
* Add (static) categories link to sidebar

* Update Makefile to support categories

* Add selection list tracer

* Update blah::blah_publish to ask for a category, if it is a post we're
  publishing
* Add helper function blah_add_category
* Add helper function blah_categorize_post

* Fix issue with blah_setup calling itself without a path
* Fix issue where licensing page would be added twice

* Add RSS support for categories