fixed newer v8 compilation
removed Clear before Dispose from newer v8 code, consider the following code:
template <class T>
void Persistent<T>::Dispose() {
  if (this->IsEmpty()) return; //Clear will trigger this
  V8::DisposeGlobal(reinterpret_cast<internal::Object**>(this->val_));
  ...
}
Merge pull request #28 from whoozle/devel

fixed newer v8 compilation