Use fenced_code and codehilite to make code work properly in markdown.

Fixes #50.
Change the teaser to cut off at the first blank line, rather than just the first line.

Fixes #59
Fixes #40.
Make selecting extra categories for an article be optional.

Fixes #47.
Sane default ordering for comments and articles.

Fixes #53
Fixes #55
Make ip, referer and user agent optional when admin adding a comment.

Fixes #54.
Sort the archive box so most recent year is at the top.

Fixes #58.
Make tags optional on articles.

Fixes #63.
Fix bug where categories showed wrong articles.

It was because we were checking string-wise contains on the category id on the article, so category 1 included
articles from category 10 etc...

Fixes #49.