Allow clearing an ACL.
Add a 'loaded' flag to ACLs.

'reset()' clears it, along with entities.

'add_entity()' sets it.
Neaten.
Use ACL-specific endpoints where feasible for buckets.

- Refactor lazy-handling of 'acl' and 'default_object_acl' using properties.

- Fetch 'ac' and 'defaultObjectAcl' via GET to specific endpoints.

- Continue to update 'acl' and 'defaultObjectAcl' via PATCH to the object's
  resource (we can't do incremental updates to ACEs).

Fixes #163.
Use ACL-specific endpoints where feasible for keys.

- Refactor lazy-handling of 'acl' using a property.

- Fetch 'acl' via GET to a specific endpoint.

- Continue to update 'acl' via PATCH to the object's resource (we can't do
  incremental updates to ACEs).

Fixes #151.
Fix save/clear ACL operations.

Borked by dropping the 'projection' qs param to PATCH.  Docs spell
a default, but then note that overriding it w/ 'full' is required
(and we need 'full' anyway, because we expect to read the resulting
ACL back).
Accomodate 'stick' ACL entries when clearing bucket's ACL.

Back-end preserves some of them even after a successful PATCH with
data .
Drop support for fetching ACLs through 'metadata' operations.

We have separate APIs for ACLs, and the 'full' option was needlessly
complex, maybe even expensive.
Add comments pointing to specific ACL-related methods.

Incorporates feedback from @dhermes.
Ensure bucket's 'acl' is marked as loaded after 'save_acl'.

Incorporates feedback from @dhermes.
Restore simplified 'clear_acl' after re-convergence.

Also 'clear_default_object_acl' for bucket.
Make 'get_metadata' comments more developer-centric.
Clarify why 'reload_metadata' uses 'projection=noAcl'.
Raise KeyError from 'get_metadata' for ACL-related fields.

Error message names appropriate ACL-specific API for the field.
Merge pull request #318 from tseaver/151-163-use_acl_endpoints

Fix #151/#163:  use ACL-specific endpoints where feasible for buckets and keys