Implement basic support for display: inline-block.

[glennw]

This still needs a lot of work, but it covers the basic
cases and improves wikipedia while passing all existing tests.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/2563

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[glennw]

@pcwalton - this is very much a prototype - would be good to know if I'm on the right track with how you were thinking of structuring it though.

[pcwalton]

nit: comment should say "inline-block items"

[pcwalton]

Looks straightforward. r=me with comment fixed.

[jdm]

failures:
---- append_style_a.html == append_style_b.html stdout ----
task 'append_style_a.html == append_style_b.html' failed at 'rendering difference: /tmp/servo-reftest-000037-diff.png', tests/reftest.rs:275

---- inline_block_img_a.html == inline_block_img_ref.html stdout ----
task 'inline_block_img_a.html == inline_block_img_ref.html' failed at 'rendering difference: /tmp/servo-reftest-000097-diff.png', tests/reftest.rs:275

[mbrubeck]

cc #225

[glennw]

Rebased (again!)

[pcwalton]

Travis seems to be acting funny so I'll merge this and take responsibility for backing out if it breaks stuff.