[SPARK-2489] [SQL] Parquet support for fixed_len_byte_array

Make it possible to read parquet files created by avro with "fixed" type columns. The underlying type in parquet is fixed_len_byte_array. Without this patch, SparkSQL will fail when it tries to read data field defined in this type. 

This pull request added a new type "FixedLenByteArrayType" mapping to the fixed_len_byte_array format.