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/)