SPARK-1383 [SQL] Improvements to ParquetRelation

Improvements to Spark-SQL's ParquetRelation:
- 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 in HiveQL
- Parquet tests now use the new API
- Parquet logging is now forwarded to Log4J
- Compression is enabled by default (default compression for Parquet
  files: GZIP, as in parquet-mr)
SPARK-1383 [SQL] SQLContext now has reset to drop tables