add -stats and -batch-size support to bench cmd
Add --batch-size to 'bolt bench'.

This commit adds a --batch-size CLI argument to the 'bolt bench' tool.
This argument will insert into Bolt in smaller batches which is a more
typical use case.

/cc @snormore
Remove bolt bench -stat.
Merge branch 'bench-batch' into moar_bench

Conflicts:
	cmd/bolt/bench.go
Merge pull request #146 from benbjohnson/bench-batch

Add --batch-size to 'bolt bench'