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.

Simply pass the script's location to escript and the root directory for all bitcask files that need to be reformatted:

```
$ escript downgrade_bitcask.erl /my/riak/data/bitcask
```