Fix issue #601 - Update AJAX example to load from Hacker News

Update Example 6 (AJAX-loading data with search) to access the Hacker News search API rather than the now-obsolete Digg API.

Some of the code that talks to HNSearch was taken from Aleks Jakulin's nice work on pull request #412 (commit d175aa14fda2a7ce4edcdbc7e6a6fc17349bcec4), but my changes to the example are more minimal. I've also improved the sorting a bit and added a sortable Date column.