Add saving mxnet model checkpoint callback

[sandeep-krishnamurthy]

https://github.com/awslabs/keras-apache-mxnet/issues/130

• Adding callback for model checkpointing in native MXNet model format.
• Allows saving MXNet model after each epoch or the best model till now in training.
• For save best model, replaces the previous best model params file.
• For save all models in all the epoch, param files is prefixed with epoch.
• Do not take file path, always saves in current working directory. For this, we need to also update the save_mxnet_model API to take the filepath, will be done in a separate PR.
• Add test cases.

@roywei @kalyac

[sandeep-krishnamurthy]

@roywei @kalyc - Ready for review.

[sandeep-krishnamurthy]

@kalyc - Thanks for your review. Addressed the comments.

[sandeep-krishnamurthy]

@roywei - Thanks for reviewing. Yes, we can add check in save_mxnet_model API. I would like to do it in separate PR. Also, I would like to introduce filepath option for allowing users to save in their directory of choice.

That is right, keras users are accustomed to pass save_best_only=True when using ModelCheckpointCallback.