Spark parquet improvements

A few improvements to the Parquet support for SQL queries:
- Instead of files a ParquetRelation is now backed by a directory, which simplifies importing data from other
  sources
- InsertIntoParquetTable operation now supports switching between overwriting or appending (at least in
  HiveQL)
- tests now use the new API
- Parquet logging can be set to WARNING level (Default)
- Default compression for Parquet files (GZIP, as in parquet-mr)

Hi @AndreSchumacher and @marmbrus , it seems we can use a hadoop config property to change this ```defaultCompression```, in ```createEmpty``` method, there is a check:
```
    if (conf.get(ParquetOutputFormat.COMPRESSION) == null) {
      conf.set(ParquetOutputFormat.COMPRESSION, ParquetRelation.defaultCompression.name())
    }
```

but it is a hadoop config property, not a spark config property, so we can not simply set this property in conf/spark-defaults.conf