Differential results

Now, if a result from a platform'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.

What this basically means is, all you usually need to specify for particular platforms is the result for the *earliest version*, plus the result for *the version where support became true* (if any). For instance, here's the `Number.EPSILON` results:
```
    tr:          true,
    ejs:         true,
    ie10:        false,
    firefox11:   false,
    firefox25:   true,
    chrome:      false,
    chrome34:    true,
    safari51:    false,
    webkit:      false,
    opera:       false,
    konq49:      false,
    rhino17:     false,
    phantom:     false,
    node:        false,
    nodeharmony: true
```
If a certain range of versions needs a note attached, that can also be specified (see these results for `__proto__`):
```
    tr:          true,
    ejs:         false,
    ie10:        false,
    ie11:        true,
    firefox11:   true,
    firefox33:   {
      val: true,
      note_id: 'fx-proto-shorthand',
      note_html: 'Firefox 33 and 34 incorrectly regard both of the following as true: <ul>'
      +'<li><code>var __proto__ = []; ({ __proto__ }) instanceof Array</code>'
      +'<li><code>({ __proto__(){} }) instanceof Function</code>'
      +'</ul>'
    },
    firefox34:   { val: true, note_id: 'fx-proto-shorthand' },
    firefox35:   true,
    chrome:      true,
    safari51:    true,
    webkit:      true,
    opera:       true,
    konq49:      true,
    rhino17:     true,
    phantom:     true,
    node:        true,
    nodeharmony: true
```

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.