Implement collapsing margin
Adjust top_offset when top margin of a element collapses with its first child
Add reftest for margin collapsing
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
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.
Add css module to use border-style
Add style field to render them in RenderContext
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
Add apply_border_style to apply each CSSBorderStyle
Update submodules
Add more test codes for border-style. And remove a file for same test