include batch() and chainable batch tests

The batch() tests are ready for use in AbstractLevelDOWN so I've included them here. You have issues with `type:'del'` and `batch.del()` that need to be sorted out to get passes, but the rest is ready to roll and looking good--you get the new chained batch syntax free of charge! (e.g. `db.batch().put('foo', 'bar').del('bam').write(cb)`)