Trigger garbage collection within Isolate::Scope

Before the idle notifications to V8 were sent without a special
Isolate entered, which results in V8 using it's default isolate
(which we don't use at all).  Hence those were pretty useless
anyways (if they were called, which was unlikely).

Besides V8 seems to not trigger the weak object callbacks if
the isolate is destroyed, hence the PHP objects we add-ref'ed
before will leak.

Therefore this removes the previous idle notification logic
and forces garbage collection from the V8Js object destructor.
Delete global PHP object on shutdown

Deleting the global object implicitly deletes all the properties,
i.e. PHP objects we previously attached to it (and for which we
increased the reference counter).

Since the following idle notifications trigger the weak callbacks,
the references on the PHP objects are finally decreased.
Merge pull request #67 from stesie/fix-garbage-collection

Fix garbage collection