Slf bulk insert shortcut2

Add the following:

1. Allow the user to specify all of the usual basho_bench config
   knobs for key distribution & size and value size & content, and
   bucket name.
2. Specify an n_value and a path to a local Riak data directory.
3. A driver to allow basho_bench to calculate preflists based on
   a 1-node Riak cluster, then write Riak KV objects directly into
   appropriate Riak vnode data directories.

It supports bitcask, eleveldb, and hanoidb.