Move the building of related posts into their own class
Move the require for related_posts to jekyll.rb
Wrap tests around Jekyll::RelatedPosts

This gives me more confidence that we're doing the right things when it
comes to both the LSI and non-LSI cases and prevents regressions.
Update the display of the LSI progress output

It now fits more in line with what the other messages display.
Also stub the building of the index

Since we don't actually use the index in getting the related posts from
the tests there's no need to build an index, which can take a long time
if the ruby bindings for the GSL library are not installed.
Merge branch 'master' into refactor-related-posts
Use Jekyll::Stevenson#formatted_topic for printing messages

Beats trying to line them up yourself. :-)
Merge pull request #1057 from mojombo/refactor-related-posts

Move the building of related posts into their own class