Make the following changes to the S3 driver:
 - Add ACL support for uploads
 - Add support for AWS tokens
 - Modify get_container to use a more efficient HEAD approach

Closes #223.

Signed-off-by: Tomaz Muraus <tomaz@apache.org>
Update CHANGES.
docs: Update upgrade notes, add a note about get_container change.
docs: Add an example of how to specify canned ACL when uploading an object in
the S3 driver.