Instead of every NativeFunction callback having to ask the Interpreter
for the current "this" value and then converting it to an Object etc,
just pass "this" as an Object* directly.
This patch introduces the Wrapper and Wrappable classes.
Node now inherits from Wrappable, and can be wrapped in a GC-allocated
Bindings::NodeWrapper object. The only property we expose right now is
the very simple nodeName property.
When a Document's JS::Interpreter is first instantiated, we add a
"document" property with a DocumentWrapper object to the global object.
This is pretty cool! :^)
