Add format downgrade script

This script downgrades Bitcask data files to the format used before
version 1.7.0 (to ship in Riak 2.0).  In that version, a more compact
encoding is used for key values, and also 2 new types of tombstones are
introduced.
Merge pull request #184 from basho/feature/add-downgrade-script

Add format downgrade script

Reviewed-by: aberghage