Mark obsolete engine versions in ES6 compat table.

All engine versions that are no longer officially maintained (at time
of commit) are marked as obsolete in the ES6 compatibility table. When
an engine is marked as obsolete, the "obsolete" class is added to each
respective table cell when building the output HTML file.
Auto-hide obsolete engines in ES6 compat table.

Result table cells (and thus columns) of obsolete browsers are hidden
via CSS. A checkbox allows for showing the hidden data.
Re-Add Firefox 28 results from #87.

Pull Request #87 did only change the (generated) index.html. This commit
adds the results to the data-es6.js and includes the generated file.
Merge pull request #88 from fbender/es6-ux3

Usability enhancements, Part 1, 2nd try (issue #75)