Perform case-insensitive field name lookups

This is really to fix the Hive SerDe, specifically `DataWritableReadSupport.init`. If a Parquet file contains fields that are not lowercase they show up as null binary columns in Hive, due to the way schema evolution is implemented. The rest of Hive does case insensitive comparisons. So the question is: should case insensitivity be part of `parquet-column` or does it need to live in `parquet-hive-storage-handler`?

Fwiw the Pig loader works just fine with mixed case column names.