Add font-awesome icons
Fix post header comments link problems

The post header comments now link to the comments sections, and only
show up if "comments: true" is put in the post's YAML front-matter.
Therefore #4 is resolved.
Remove post header categories
Add post pager

The post pager code is courtesy of
http://erjjones.github.io/blog/How-I-built-my-blog-in-one-day/ in the
section labelled "Paging".
_config.yml and post pager cleanup

_config.yml now excludes README.md as it doesn't need to be touched at
all. The css of the post pager arrows was tidied up to remove the bad
looking followed-link feel, and changed to something more fitting.