Removed unnecessary local handle creation.

This should speed up handle creation on old Node.

On another note: Would it be worthwhile to make `NanNew<T>(v8::Persistent<T>)` check for possible weakness and just do a typecast if the handle is not weak? Only reason I don't like it is because it then would not return a *new* handle, which the name implies it should. Is there any performance to gain here in the first place?
These changes arguably belong in a separate commit, they're not related to the new bool-to-Handle method.