  This paper has been withdrawn as we discovered a bug in our tensorflow
implementation that involved accidental mixing of vectors across batches. This
lead to different inference results given different batch sizes which is
completely strange. The performance scores still remain the same but we
concluded that it was not the self-attention that contributed to the
performance. We are withdrawing the paper because this renders the main claim
of the paper false. Thanks to Guan Xinyu from NUS for discovering this issue in
our previously open source code.
