Add "attributes" argument to boto.dynamodb2.table.Table.batch_get

The DynamoDB API supports an `AttributesToGet` parameter for the [BatchGetItem operation](http://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_BatchGetItem.html), but [boto.dynamodb2.table.Table.batch_get](http://boto.readthedocs.org/en/latest/ref/dynamodb2.html#boto.dynamodb2.table.Table.batch_get) does not yet support it.

This adds an `attributes` argument similar to the one currently supported by [Table.get_item](http://boto.readthedocs.org/en/latest/ref/dynamodb2.html#boto.dynamodb2.table.Table.get_item) to `batch_get`.  It also includes the one-line fix from #2272.