Add two sections (experiments and model released) to README.md for DS2.

1. Add options to enable and disable RNN weights sharing.
2. Set rnn_layer_size to 2048 by default.
3. Revert back the striding steps of 1st conv layer from 2 to 3.
4. Revert back to BRelu.

Above follows DS2 papers.

Rename self.local_data to self._local_data in class DataGenerator.

Fixed a serious error of bidirectional simple rnn for DS2.

Fix a typo caused exception for audio_featurizer.py.