Cxflow-tensorflow

Noteable features:
- Add support for batch normalization, regularization and custom variables initialization
- Add `InitLR` hook
- Make `bin_stats` more robust

First **cxflow-tensorflow** release intended for public audience.

See https://tensorflow.cxflow.org for more information.

Major features
- **cxflow** compatible `BaseModel` for Tensorflow
- multi-gpu training with `n_gpus` configuration
- **cxflow** hooks (https://tensorflow.cxflow.org/cxflow_tensorflow/cxflow_tensorflow.hooks)
- `WriteTensorBoard`
- `DecayLR`