Fix to avoid attribute name clash between Document/HTMLDocument

Use children() instead of traverse_preorder(), and avoid having
GetHead() in both Document and HTMLDocument.

Closes #1465.
auto merge of #1472 : brunoabinader/servo/document-head, r=jdm

Use children() instead of traverse_preorder(), and avoid having
GetHead() in both Document and HTMLDocument.

Closes #1465.
Update UA stylesheets for lists
Upgrade to latest Rust.
auto merge of #1473 : metajack/servo/rustup-20131219, r=jdm
auto merge of #1478 : recrack/servo/useragentcss, r=SimonSapin

#1232

Servo UA (http://www.w3.org/TR/CSS21/sample.html)

This is UA css from http://mxr.mozilla.org/mozilla/source/layout/style/html.css and http://dev.w3.org/csswg/css-lists/#ua-stylesheet

```
ul margin-left: 40px -> padding-left: 40px
ol margin-left: 40px -> padding-left: 40px
```
Remove needless code.

This `OK(())` should be propagated from `Element.set_attribute()`.
Split up the utility function to make internal attribute's namespace.
Enable only remove operation for Document::update_idmap().
Add the comment to explain the dual declaration.
Implement Element.removeAttribute()/removeAttributeNS().
auto merge of #1448 : saneyuki/servo/attr, r=jdm
Remove unused imports.
auto merge of #1481 : Ms2ger/servo/unused-imports-2, r=metajack
Initial implementation of JSManaged for Events.
Make reflection work for new JSManaged types.
Generate base and derived traits and implementations for DOM types automatically.
Generate traits for casting up and down the DOM hierarchy.
Force Event and its children to use the Box representation.
Make event-related tests pass again by ensuring we're unwrapping to the correct types.