Add Keras ImageNet training example and LearningRateScheduleCallback

Add -stdlib=libc++ to CFLAGS for MacOS

Modify LRWarmupCallback to scale LR from `initial_lr / hvd.size()` to…

… `initial_lr`

Bugfix: Initialize MetricAverageCallback variables

Add support for boolean broadcast() and allgather()

First-class Keras support

Add support for new tf.sysconfig.get_compile_flags() and tf.sysconfig…

….get_link_flags()

Add requirement for TensorFlow 1.1.0+

Bugfix: Add grad=None check in DistributedOptimizer

Improved integration with NCCL