hadoop.io.Lz4Codec.py import and location problems

[lewismc]

A few issues when looking into Lz4Codec.py

1. Why is it not also located in the hadoop.io.compress package alongside hadoop/io/compress/BZip2Codec.py, hadoop/io/compress/GzipCodec.py and the default codec hadoop/io/compress/ZlibCodec.py?
2. The statement import lz4_raw cannot be resolved. That module is absent. I propose we use the popular lz4 library which would offer import lz4.frame and then `lz4.frame.decompress(...)