Filtered Reader Implementation, Avro Specific Support

As discussed on groups I have implemented:

- Ability for a filter to be supplied to a ParquetReader to enable it to pull out only a select set of records based on a predicate, page specifier or both.

- Underlying above skip() operation has been added to Plain and Dictionary readers.

- Support for AvroSpecific, in addition to current AvroGeneric support.

- Tests for above.