Added support for series.

- Each post is now part of 0 or more series, and a list of all of the series
  can be browsed, as can a list of the posts in a given series.
Changed name scheme for files in content/post; added some posts.

- Previously, post files in content/post had the following name format:
  yyyy_mm_dd_Name_of_Post.md
  Now, they're lower cased:
  yyyy_mm_dd_name_of_post.md
- Added posts from the Soylent series.
Added better support for navigating through and browsing posts.