Subdivide xs range into xs-A and xs-B, to introduce a fully BC 480px break point.

See https://github.com/twbs/bootstrap/issues/10203#issuecomment-36443874
An additional breakpoint at 480px has been requested heavily, but there have been concerns about BC, especially regarding features that regard those ranges as min-max intervals instead of open-end intervals.

Instead of redefining existing ranges, this PR introduces subdivisions of the old ranges:
* xs = 0..767 or 0..* (no change)
* xs-A = 0..479 or 0..*
* xs-B = 480..767 or 480..*
* sm = 768..991 or 768..* (no change)
* md = 992..1199 or 992..* (no change)
* lg = 1200..* (no change)

E.g. you can now write

    <div class="row">
      <div class="col-xs-B-6 col-sm-4 col-md-3">..</div>
      ..

with the following effect:
* 0..479px: 1 column
* 480px..767px: 2 columns
* 768px..991px: 3 columns
* 992px..*: 4 columns