ParquetFileViewer only works on world-readable files

[erwa]

As mentioned in #114, the ParquetFileViewer tries to view every file as azkaban, meaning it can only view files owned by azkaban, with the group set to azkaban, or world-readable. This means users will not be able to view their Parquet files through the HDFSViewer unless their Parquet files are world-readable.

[erwa]

Proposed solution by @davidzchen, mentioned in #115:

We would need to add an API to the AvroParquetReader that allows us to pass in an InputStream similar to what Avro allows us to do with DataFileStream, which takes an InputStream and a GenericDatumReader.