Make ComputedStyle fields private and add getters.

This isolates layout code from upcoming refactoring in properties.rs.mako.
Un-disable and fix some warnings in the style crate.
Remove CowArc and use the new Arc::make_unique() method instead.
Submodule updates for #2137
Update submodules for #2492
auto merge of #2495 : mbrubeck/servo/2137-submodule-updates, r=larsbergstrom
Derive Clone for ChildFrameTree instead of implementing it manually.
auto merge of #2496 : zwarich/servo/deriving-clone, r=jdm
Request header and postdata support for XHR
auto merge of #2493 : Manishearth/servo/xhr-post, r=jdm

Blocks #2282
Remove unnecessary pubs from script_task
auto merge of #2478 : SimonSapin/servo/style-getters, r=metajack

This isolates layout code from upcoming refactoring in properties.rs.mako.
Make some scary things in the workqueue module private
auto merge of #2499 : mbrubeck/servo/unpub, r=metajack
Make the workqueue's type parameter names less terrible
Don't overwrite the "Host" request header

Fixes broken requests caused by #2493.
auto merge of #2500 : bjz/servo/workqueue, r=pcwalton

cc. @pcwalton
Prefix fragment types in inline.rs with `Inline`

This is to reduce confusion once we rename `servo::box_::Box` to `servo::fragment::Fragment`.
Convert usages of `Box` in type identifiers to `Fragment`
Rename functions, fields and adjust comments to use `fragment` instead of `box`

I tried to make sure to keep 'box' where it made sense, and alter comments so they still make sense.