src: update MakeCallback() function prototype

Make it possible to invoke MakeCallback() on a v8::Value but only for
the variant that takes a v8::Function as the thing to call.

The const char* and v8::String variants still require a v8::Object
because the function to call is looked up as a property on the receiver,
but that only works when the receiver is an object, not a primitive.
src: fix MakeCallback() handle leak

Create a new HandleScope before looking up the object context with
v8::Object::CreationContext(), else we leak the Local<Context> into
the current HandleScope.

That's relatively harmless unless the HandleScope is long-lived and
MakeCallback() is called a lot.  In a scenario like that, we may end
up leaking a lot of memory.

What is unfortunate about this change is that we're trying hard to
eradicate the node_isolate global.  Longer term, we will probably have
to change the MakeCallback() prototype to one that requires an explicit
v8::Isolate* argument.
src: add tracing.v8.on('gc') statistics hooks

Add a new 'tracing' module with a v8 property that lets the user
register listeners for gc events.  The listeners are invoked after
every garbage collection cycle with 'before' and 'after' statistics.
Useful for monitoring tools that want to keep track of memory usage.
src: add v8.getHeapStatistics() function

Add a one-to-one binding to v8::GetHeapStatistics().  Returns info on
the current state of the JS heap, like total size and amount used.
doc: document the tracing api