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.
Maybe so, but this test is not used. This tests seem old ones, and if we fix this, we'll need Riakc mock behind Riak client and several fixes.