Resolving inter-article navigation, adding tests, making magic happen

This is part of the fix for the horrifically-huge issue #41 and resolves the issue where navigating between articles in the same section was not behaving as it does in Lungo.

It also fixes some navigation issues when transitioning backwards/forwards and brings in some extra special tests to test for it.

Other highlights are:

1. Updated the included version of Lungo to 2.1
2. Included a 'local' copy of the Lungo theme that uses fonts downloaded from Google

I'll be merging this myself; the PR is mostly to capture the changes and allow an easy reference from the issue.