Implement support for style attributes.

If a style attribute is given for a node, it is parsed and attached to the
Element. When selector matching runs on the Element, the style attribute's
stylesheet is passed in.

Fixes #86.
auto merge of #678 : metajack/servo/style-attributes, r=jdm

If a style attribute is given for a node, it is parsed and attached to the
Element. When selector matching runs on the Element, the style attribute's
stylesheet is passed in.

Fixes #86.