Properly account for relative CSS units in borders, margins, padding, and widths.
Renaming and refactoring `spec_or_default`.

`spec_or_default` is now `specified_or_default` and `specified_or_zero` was
added to handle the most common case.
Brace multi-line match patterns per coding style.
Rename `cb_width` to `containing_width`.
Rename variables for clarity.
auto merge of #602 : metajack/servo/relative-bpm, r=pcwalton

...d widths.