Differential results

Now, if a result from a browser's version is not different from that of
its previous version, the result need not be recorded in the data files.
Instead, build.js will copy the result over automatically.
I've thus cleared out all unnecessary lines from data-es6.js. This has
resulted in a large diff, but you may observe the es6/index.html diff to
see that little has changed w/r/t the output results.

Note: browsers are determined to be the same version by comparing their
"full" string. As a result, certain older Chrome versions now have the
"full" string "Chrome, Opera", even though they predate Opera 15.

ALSO:
* Fixed a few Chrome and Firefox results as per #239 (and closing it).
* Fixed some older Firefox results that referenced "firefox28" instead
of "firefox27" (which comprises both 27 and 28).

This closes #224.