Fixed missing display name when new users are created.

The problem was caused by passing `ID`, instead of `id`, to `get_user_by` function.
Another thing improved is to check current user so that we can get two different
messages depending a user creates another user or a non logged-in visitor trying to
register himself.

This fixes issue #31.
Inline doc fix, use display name with user login fallback in log message
Adding user role information to log message
Use role labels instead of slugs in log message
Merge pull request #35 from x-team/issue_31

Fixed missing display name when new users are created.