Add background-image property
Impl background-image support.
Add background test file
Add a reftest for background-image
Support base_url in stylesheet
auto merge of #1631 : recrack/servo/background-image, r=SimonSapin

for #777 #800

- support(http://www.w3.org/TR/CSS21/colors.html#background)
 - background: url(foo.png)
 - background: url(data:image/png;base64...)
 - background-image: url(foo.png)
 - background-image: url(data:image/png;base64...)

- not support(http://www.w3.org/TR/css3-background/)
Reduce code duplication in CSS parsing by passing base_url everywhere.
Add a test for the base URL of external stylesheets.
auto merge of #1711 : SimonSapin/servo/background-image, r=pcwalton
box: Use macros to reduce code duplication
Always use the border-box for Box.position.size

Fixes #1674.
reftest: Allow free-form comments
Add a reftest for #1674
Add a simple iframe reftest (disabled for now)

Disabled due to iframe crashiness.
Rename Box.position to border_box
auto merge of #1699 : kmcallister/servo/border-box, r=pcwalton

I think this is the only change necessary to make block and inline boxes consistent.  But I'm finding it hard to test due to other bugs (#1696, #1697, #1698).