ParquetSharp 2.0.2 Beta 2

• Upgraded to Arrow 0.15.1.
• Exposed WriterPropertiesBuilder.CompressionLevel (new feature from Arrow 0.15).
• Use Lz4 as the default compression argument when creating a writer, instead of Snappy. Lz4 has empirically been shown to both be faster and compress better than Snappy. Be aware that not all Parquet implementations may support Lz4 yet.
• By default close streams when managed stream wrapper classes are closed (PR #76).