Improves the categories.html templates

This is just a suggestion, but I thought the categories page could look
nicer.
Merge branch 'categories' of https://github.com/magnunleno/pelican-bootstrap3 into magnunleno-categories

* 'categories' of https://github.com/magnunleno/pelican-bootstrap3:
  Improves the categories.html templates
Prettified tags as well