Implement custom notification on weak reference garbage-collection

[camilleoudot]

In our application, we need to be notified when some specific V8Value references that have been made weak are garbage-collected by V8, so that we can release some associated object references in the java world too.

This patch adds the ability to optionally set a custom observer on any weak reference. The custom handler is memorized in the V8.v8WeakReferences map, and not as a property of the V8Value, to avoid the cost of an extra field for all the V8Value objects.

[whoozle]

any chance of merging it? we need it too 😢 👍