Move content tests to src/test/content for consistency
CONTRIBUTING: link to testing guide
auto merge of #1859 : kmcallister/servo/move, r=jdm

And update `CONTRIBUTING.md`.
Use Option::or_else in eventdispatcher::dispatch_event.
auto merge of #1863 : saneyuki/servo/1841, r=metajack

fix #1841
Move JSVal into the jsval module.
Use FooValue() functions.
Use the JSVal member functions to replace JSVAL_IS_* and JSVAL_TO_*.
Update rust-mozjs to include fixes for GC crashes.
auto merge of #1868 : Ms2ger/servo/JSVal, r=jdm

Requires https://github.com/mozilla-servo/rust-mozjs/pull/54.