Allow asynchronous delete support

[jgeewax]

If I want to delete all the objects in a bucket, I have to figure out the threading on my own to make this quick... I don't care about the response for the most part...

for key in bucket:
  key.delete(async=True)
  # or key.delete_async()

While we're at it, it might be nice to have an "empty bucket" method:

bucket = connection.get_bucket('...')
bucket.delete_all_keys()

[tseaver]

We don't do any other async programming within the library: introducing it would inject a lot of complexity.

For the second, wouldn't the following do?

bucket.delete_keys(bucket.get_all_keys(), on_error=lambda *key: None)

[jgeewax]

The use-case is basically deleting a big chunk of keys, so if we can do that with

bucket.delete_keys(bucket.get_all_keys())

then I'm happy and you can resolve this...