Properly render negative score updates.

The old code displayed score updates using the "score-addition" class,
but only for positive updates; negative updates (possible only in
variants) didn't get that animation.
Fix .scss files to work with Sass 3.4.5.

Two bugs here, relative to Sass 3.4.5 (Selective Steve).
The first bug, reported at
https://github.com/dsociative/2048/commit/80258ad81716fd0dc263a3b0e154c7624afe1a0e
is that Sass apparently requires interpolation braces #{} around
a mixin parameter used on the left side of a colon.

The second bug, visible at that same commit, is that when you
refer to an unbound variable inside a mixin, Sass looks it up
in the global scope rather than in the local scope of the "caller"
of the mixin. For example,

    @mixin foo() { bar: $bar; }

    $bar: 1;
    a {
      $bar: 2;
      @include foo();
      baz: $bar;
    }

gives `a { bar: 1; baz: 2; }`.

According to the Sass maintainers, this is by design. The fix is to
explicitly mutate that global variable via `!global` rather than
trying to get the mixin to use a local variable.
https://github.com/sass/sass/issues/1474
Refactor HTMLActuator to properly render variant tiles >1000.

The motivation for this patch is that I want to add a variant
merge rule that generates four-digit numbers that aren't powers of
two. The old code styles such numbers with too big a font size;
e.g., a tile with value 1234 would be styled with class "tile-1234",
which doesn't exist, and then it would render sloppily. I believe
a friendly default behavior is to style such tiles with "tile-1024".

I'm also leaving the door open for variants to style the color of
the tile and its font size using two (or more) separate CSS classes:
e.g., ["tile-red","tile-4digits"].

No intended functional change.
Merge branches 'AppearanceClasses', 'MergeRules', 'Sass345' and 'ScoreDecrements' into 2048-sqrt
2048-sqrt: Add "Nth root" tiles.
Add "multiply" tiles too.
Merge branch '2048-sqrt' into gh-pages

Conflicts:
	index.html