Add post description; fix related-post filter bug.

_includes/post_summary.html
	-A post in the "related" section of another post was
	occasionally inserted in full as raw markdown, presumably due to
	Jekyll's page compilation order.
	-Fix by force-compiling to HTML with `markdownify`.

_posts/2014-10-01-prime-number-spirals.md
	-Add post description YAML variable.