JC: Support vanilla multi-object delete

[johnchapin]

This PR adds basic support for multi-object delete.

user=> (require '[aws.sdk.s3 :as s3])
nil
user=> (s3/delete-objects aws-credentials "my-bucket" ["foo" "bar" "baz"])
{:objects ({:delete-marker-version-id nil, :delete-marker? false, :key "bar", :version-id nil} {:delete-marker-version-id nil, :delete-marker? false, :key "foo", :version-id nil} {:delete-marker-version-id nil, :delete-marker? false, :key "baz", :version-id nil})}
user=> (s3/delete-objects aws-credentials "my-bucket" ["foo" "bar" "baz"] {:quiet true})
{:objects ()}

I didn't attempt to handle it here, but if there are opinions or precedent on dealing with request limits (for example, the 1000 object limit for multi-object delete), I'm happy to address them. At the moment, my application code is handling the partitioning of keys to respect the object limit, but that logic might have a place in this library instead.

Another potential TODO item is handling multi-object delete for versioned objects.

[johnchapin]

Whoops - I see this largely duplicates the effort of #70, which dates from December.