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)