Add security to Riak

This PR implements all the riak_core bits of security, that is, a place security information is stored and modified and APIs for checking a user has a particular set of permissions. riak_core applications are expected to use these APIs themselves, which is why there's no actual security checking done anywhere in this PR. The riak_kv and riak_api repos are where that happens.

See also basho/riak#355