Major refresh

- New layout framework: Implements purecss.io. Site is now properly
responsive, including images and video embeds.
- Tagging: Resolves #1 and #2, tags now displayed on all individual
entry indexes and the archive page. Also implements indexes for each
tag.
- Better Flickr integration: Resolves #3 by gracefully failing when a
Flickr image is not found.
- Better image css: Inline images now using kramdown to assign a css
class `.pure-img`, most images should now be responsive.
- Entry cleanups: Tidied tags for a large number of entries, tidied
youtube and vimeo embeds, tidied inline images wherever possible.