Allow union types to be used as return values
auto merge of #2573 : Manishearth/servo/union-return, r=Ms2ger

Blocks #2559
Make the bindings make target work on all platforms.
auto merge of #2614 : saneyuki/servo/make, r=jdm

Fix #2583
Use RefCell for mutability of Element::style_attribute.
auto merge of #2615 : saneyuki/servo/refcell, r=Ms2ger

related: #2513
Support GlobalEventHandlers.onclick.