style: Move presentational hint synthesis into `legacy.rs`.
style: Parse the legacy `border` attribute per the legacy HTML specification.

Additionally, this patch cleans up some miscellaneous formatting issues
and refactors files in `layout/css/` somewhat to eliminate needless
levels of indirection. It also fixes our handling of presentational
hints that only apply if border is nonzero.
style: Parse the legacy `bgcolor` attribute per the HTML5 specification.

Additionally, this patch cleans up some miscellaneous formatting issues.
style: Implement basic column spans.

This patch provides some of the groundwork for column spans greater than
1. It implements the column-span CSS property (prefixed so as not to be
exposed to content) as well as the corresponding colspan attribute;
although the former is not well-specified outside of CSS multi-column
layout, INTRINSIC refers to it. Although width is distributed to
spanning columns, they do not yet contribute minimum and preferred
widths; this will be implemented in a follow-up.

Additionally, this patch cleans up some miscellaneous formatting issues
and improves the handling of table rowgroups.
style: Implement quirks mode rules.
style: Address review comments relating to `bgcolor` and column spans
auto merge of #4289 : pcwalton/servo/hacker-news, r=SimonSapin

This patch provides some of the groundwork for column spans greater than
1. It implements the column-span CSS property as well as the
corresponding colspan attribute; although the former is not
well-specified outside of CSS multi-column layout, INTRINSIC refers to
it. Although width is distributed to spanning columns, they do not yet
contribute minimum and preferred widths; this will be implemented in a
follow-up.

The parsing for the legacy bgcolor and border attributes is
implemented according to the WHATWG HTML specification.

Additionally, this patch cleans up some miscellaneous formatting issues,
refactors layout/css somewhat to eliminate needless levels of
indirection, and cleans up the handling of table rowgroups.

New Hacker News screenshot: http://i.imgur.com/hnl2a7E.png