Make Attr::value private.
Simplify Element::set_attribute for its remaining callers.
Added Node's query_selector_all helper function
Implement support for ParentNode's querySelectorAll
Added ParentNode's querySelectorAll content tests
Update submodules and ~str/String, to_owned/to_string, & cast/mem to get android building.
Use interior mutability for MouseEvent.
auto merge of #2632 : brunoabinader/servo/document-queryselectorall, r=Ms2ger

Spec:
http://dom.spec.whatwg.org/#dom-parentnode-queryselectorall

Closes #851.
Use internal mutability for Document::idmap.
Pass immutable self to Node::{SetNodeValue, SetTextContent}.
Pass immutable document to parse_html.
Cleanup a mutable variable in dispatch_event.
Pass immutable self to HTMLBodyElement::SetOnunload.
auto merge of #2640 : Ms2ger/servo/derefmut, r=jdm
auto merge of #2628 : luqmana/servo/up, r=mbrubeck
Update layers submodule for AMD GPU fix.
Add rust-phf to servo build.
auto merge of #2647 : glennw/servo/issue-2462, r=metajack
Prepare makefile.backup for snashot changes
auto merge of #2648 : Manishearth/servo/makefile.backup, r=jdm