Support newer posts (up to 2013)

 - support urls in http://lesswrong.com/r/lesswrong/lw format

 - when caching url contents, hash filenames that would exceed maximum
   filename length

 - don't create image files with weird extensions

 - change date range in title
Add Previous and Next lables to sequence navigation links
Add sequence "Highly Advanced Epistemology 101 for Beginners"