Use DOMString more consistently.
auto merge of #1690 : Ms2ger/servo/DOMStrings, r=jdm
auto merge of #1691 : kmcallister/servo/linked-failure, r=pcwalton

We probably leak some threads and resources, e.g. when the script task crashes and doesn't get a chance to send layout data back to layout to be deallocated.

Not tested with iframes yet.
Move DOMString into servo_util.
Add str.rs.
auto merge of #1694 : Ms2ger/servo/move-DOMString, r=SimonSapin
box: Use macros to reduce code duplication
Always use the border-box for Box.position.size

Fixes #1674.
reftest: Allow free-form comments
Add a reftest for #1674
Add a simple iframe reftest (disabled for now)

Disabled due to iframe crashiness.