Fix infinite loop bug.
Handle doctype.textContent correctly.

I confused Doctype and DocumentFragment nodes when implementing.
Fix unused import warnings.
auto merge of #890 : Ms2ger/servo/textcontent-getter-bug, r=jdm

I confused Doctype and DocumentFragment nodes when implementing.
auto merge of #891 : Ms2ger/servo/unused-import-warnings, r=jdm
Make the global object be the Window object. Fixes #833.
auto merge of #759 : ILyoan/servo/collapsing, r=metajack

A part of implementation for CSS 8.3.1 Collapsing margins.
auto merge of #883 : jdm/servo/newglobal, r=kmcallister

Requires https://github.com/mozilla-servo/rust-mozjs/pull/34.
auto merge of #882 : sammykim/servo/infinite-loop, r=metajack

To reproduce:

```html
<span style="font-size: 128px">abc</span>
```
Add some dom node tree implementations
Add node.removeChild
Add AncestorIterator by jgraham
Add node.appendChild