Introduce ScalaJSPackfiles and generalize JSClasspath generation
Fix #441: Add Node.js environment
Rename preoptimize to fastOpt and optimize to fullOpt
Fix #458: Correct DCE info for default params in constructor exports
Fix #459: Add class name to pseudo-member <exported>

If a subclass exports different members than its superclass, the DCE
info of the exports of the superclass were lost since the subclasses
<exported> member was (in DCE's terms) overriding the superclasses
<exported> member. And since (in some cases) the superclass was never
instantiated as such, the superclasses <exported> member was not
reachable. It's dependencies got therefore eliminated.
Fix #460: Add missing DCE info for reflective call proxies on Object
Separate and deactivate test for #461
Fix #463: Add java.util.concurrent.Executor
Fix #462: Add java.util.concurrent.ExecutionException
Fix #267: Support to run and test optimized code (with Node.js)