Add support for schema projection in Avro

This commit updates the AvroReadSupport and AvroParquetInputFormat
classes to allow users to request a schema projection.
Merge pull request #96 from massie/master

Add support for schema projection in Avro
Add support for predicate pushdown in ParquetInputFormat

This commit allows users to define an UnboundRecordFilter to be
used when reading Parquet records from the ParquetInputFormat.
Merge pull request #98 from massie/matt-pushdown

Add support for predicate pushdown in ParquetInputFormat