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.
Better to have `riak_cs_user_test.erl` ?