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.