Added SQS::Message batch delete operation

You can now receive and delete messages in batches. This is much more efficient than deleting the messages one at a time.

```ruby
messages = queue.receive_messages(max_number_of_messages: 5)
# do something with messages ...

# delete the batch of messages, makes 1 request
messages.delete

# delete messages one at a time, makes n requests
messages.each(&:delete)
```