[Patch] Add support for parsing writing-mode property v1

This patch is intended to support the parsing and storage of the "writing-mode" property, which is one of the building blocks for vertical text in css (support for which is currently required by the HTML5 track element and obviously is a pretty important localization feature -- and just simply looks cool, too).

The layout side of things can be dealt with elsewhere, but first and foremost is getting libcss to support the property.

A link to the current draft is here http://dev.w3.org/csswg/css-writing-modes/ with the writing-mode property in particular described in http://dev.w3.org/csswg/css-writing-modes/#writing-mode

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It is quite likely that there are things substantially wrong, including the lack of test data (I haven't figured out how to get the test suite to even build and run yet, `make test` seems to fail substantially here.)

So some review would be good, I'd like to get this approved by netsurf devs before trying to squeeze it into mozilla-servo.

Any and all feedback is much appreciated, thanks a bunch.