Refactor/cs user

Moving user-related functions from `riak_cs_utils` to `riak_cs_user`, like `riak_cs_bucket` and `riak_cs_manifest`. Module-per-record code structure will bring more visibility about Riak CS data structure.

This is preliminary refactoring for CS-side caching of user records - while I believe this is useful regardless of usefulness of caching. Discussion and description will follow this pull request. Will be cherry-picked to develop branch.
(not related this refactoring...)

The name and return type are rather general. `riak_cs.hrl` has type `acl_owner` and `acl_owner3`.

```
%% acl_owner3: {display name, canonical id, key id}
-type acl_owner3() :: {string(), string(), string()}.
-type acl_owner() :: acl_owner2() | acl_owner3().
``` 

These are not so restrictive but more explanatory (at least for me :) ).