Draft: Acl class design for supporting S3 ACL

This comes from previous work to achieve real S3's ACL model.
Previous discussion is here. (https://review.openstack.org/#/c/122029/)

This supports ACL class and its related model (e.g. Grant, Grantee)
with a feature to translate between a python class and an ElemntTree for
XML used for S3's request and response.

The model we need to disscuss is as follows:

AccessControlPolicy:
    Owner:
    AccessControlList:
        Grant[n]:
            (Grantee, Permission)

It comes from official S3 model overview. Please see official
documentation here (http://docs.aws.amazon.com/AmazonS3/latest/dev/acl-overview.html)
in detail.

TODO:
A stuff to translate between a python class and JSON format used for
implementations to achieve ACL handling at backend swift.

Change-Id: Ic6765f2a530caba6e22d5805323e31eb3d3013e7