add new codec option for compression in Spark-Tensorflow connector

[vgod-dbx]

With #125, it became possible to output gzipped TFrecords by setting spark.hadoop.mapreduce.output.fileoutputformat.compress in the global SparkConf.
However, there's no way to only enable compression for individual DataFrame outputs.

This PR adds a new option codec to the Spark-Tensorflow connector for enabling compression in individual DataFrameWriter. With this, we don't need to set spark.hadoop.mapreduce.output.fileoutputformat.compress globally anymore.

Sample usage:

(
  dataframe
  .write
  .format('tfrecords')
  .option('codec', 'org.apache.hadoop.io.compress.GzipCodec')
  .save('sample.tfrecord.gz')
)

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[vgod-dbx]

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[jhseu]

@skavulya Mind doing a code review?

[skavulya]

@jhseu Sure, I'll review it. Thanks!

[skavulya]

@vgod-dbx Thank you so much for the contribution. It looks good. Please add a description and example usage of the codec option to the README under the features section before merge.

[vgod-dbx]

@skavulya README updated! Thanks for the review.

[skavulya]

@vgod-dbx Thanks! Looks great.
@jhseu The PR is ready for merge

[eggie5]

@vgod-dbx what version did this make it into? I'm on 1.13.1 and it seems to ignore the codec option...

[acastelli1]

Hi it seems that the codec is ignored actually