Support & nesting and (double-)nested media queries

Example:

  .visible {
    &.visible-sm {
      @media screen { color: blue; }
    }
  }

Not the most beautiful code on the planet but does the trick.
Always break lines after commas in identifiers

Upstream lessc does it and we want to align.
Fix url("..") whitespace formatting regex.

Missed some cases.
Add missing whitespace for &
Fix '&' multiplication

Let'S do it by example:

  .ident_a, .ident_b {
    & + & + & { color: red; }
  }

We have to generate all permutations of the parent identifier list and
the amount of ampersand child identifiers. In the above case, the
exploded identifier list has ident_count**ampersand_count entries (8).

As a bonus, we get the same identifier sort order as lessc does.

Conflicts:
	lesscpy/plib/identifier.py