Better support for permissions-restricted (IAM) environments on AWS

This addresses two issues I filed ([LIBCLOUD-497](https://issues.apache.org/jira/browse/LIBCLOUD-497), [LIBCLOUD-498](https://issues.apache.org/jira/browse/LIBCLOUD-498)).

It adds:
* ACL override support for S3 via `extra={'acl': '...'}`
* Support for a `token=` keyword argument on S3 driver (and can be easily added to other AWS drivers) to include the AWS Security Token param/header, which is required when using IAM role-provider credentials or other temporary AS credentials.
* Makes `get_container` on S3 no longer call `list_containers` and otherwise work correctly in an environment with highly restricted permissions. This is a (minorly) backwards incompatible change as `get_container will no longer load the creation time of the bucket. This is not of huge importance, but should be mentioned in the release notes.