Refactor the whole data preprocessor for DeepSpeech2 (e.g. re-design classes, re-organize dir, add augmentation interfaces etc.):

• Refactor the data preprocessor with newly added classes, e.g. AudioSegment, SpeechSegment, TextFeaturizer, AudioFeaturizer, SpeechFeaturizer etc.
• Add data augmentation interfaces and classes e.g. AugmentorBase, AugmentationPipeline, VolumePerturbAugmentor etc., to make it easier to add more data augmentation models.
• Separate normalizer's mean-std computing from DataGenerator. Add FeatureNormalizer. -
• Add an independent tool compute_mean_std.py for users to create mean_std file before training.
• Re-organize data directory into datasets and data_utils.
• Test for convergence.
• Add module, class, function docs.
• Update README.md.