PARQUET-96: fill out some missing methods on parquet.example classes

I'm slightly embarrassed to say that we use these, and we'd really like to stop needing a fork, so here we are.

Author: Colin Marc <colinmarc@gmail.com>

Closes #59 from colinmarc/missing-group-methods and squashes the following commits:

af8ea08 [Colin Marc] fill out some missing methods on parquet.example classes