Fix link in README
auto merge of #1753 : kmcallister/servo/readme-link, r=SimonSapin
Add empty text check for first child of pre in innerHTML.(fixes #1743)
Move Traceable, trace_reflector from utils.rs to trace.rs.(fixes #1748)
auto merge of #1758 : lpy/servo/issue1748, r=Ms2ger

see #1748
Add test
auto merge of #1759 : lpy/servo/issue1743, r=Ms2ger

see #1743
Move ID management on binding to and unbinding from tree into Element.
Cleanup Element::set_attribute.
Pass a non-Option DOMString to after_remove_attr.
Call after_remove_attr before removing the attribute, and rename it before_remove_attr.
Call unregister_named_element rather update_idmap from before_remove_attr.
Only register the new ID from after_set_attr, and remove update_idmap.