Inform V8 of external allocations.

Inform V8 of external allocations.

This is sloppy: after each ObjectWrap allocation the user needs to
call ObjectWrap::InformV8ofAllocation(). In addition each class deriving
from  ObjectWrap needs to implement the virtual method size() which should
return the size of the derived class. If I was better at C++ I could
possibly make this less ugly. For now this is how it is.

Memory usage looks much better after this commit.