PARQUET-12: Add specs for new logical types.

This adds the new logical types from #3 to the LogicalTypes.md specification.

Author: Ryan Blue <rblue@cloudera.com>

Closes #5 from rdblue/PARQUET-12-add-new-type-docs and squashes the following commits:

be414fe [Ryan Blue] PARQUET-12: Add specs for new logical types.