New default theme.

- based on Bootstrap 3.
- themed using http://bootswatch.com/readable/
- redesigned archive page from scratch.
- if rss_tags is defined in conf.yaml, links to these
tags are shown in the main navigation.
- microformats used in post pages (h-entry, dt-published,
h-card, ...)
- responsive layout.
- numerous bug fixes.
Modified default conf.yaml.example
updated post.example.md

- removed type:
- added abstract: