SourceMap: Selector extraction overhauled.
With the way we were extracting the selectors
before, some nested scenarios were failing.
With these changes, each subset of selector
now corresponds to its selector.

Secondly, SCSS' source maps is buggy.
Either that or they are doing it differently
than LESS and CoffeeScript. Nonetheless,
an issue is filed at hcatlin/libsass#324.
Meanwhile, a post-process(ugly hack) is deployed
to bring harmony/uniformity in decoded maps from
various compilers.