Add Avro specific support to AvroParquet{Input,Output}Format

This commit generalizes AvroIndexedRecordConverter, AvroReadSupport and
AvroRecordMaterializer to enable Parquet to read/write Avro specific
objects.
Merge pull request #94 from massie/avro-specific

Add Avro specific support to AvroParquet{Input,Output}Format