Changes for UTF-8 support in users and groups (plus lower-case grant syntax)

Tied to forthcoming PR for `riak_core`.

* grant/revoke no longer require all-caps
* Switch to a user name of "Hindi" in Devanagari to test protobuf unicode
* Turn "user" into a variable in http_security so we can test utf-8 usernames when we fix HTTP support for authentication of same

/cc @Vagabond