Add  Server API method: UserInfo

Add a method to the UserManagerAPI which takes a username and returns a struct of user information (username and display name at this stage).
Please separate the info from the result.

    type UserInfo struct {
        Username string `json:username`
        DisplayName string `json:display-name`
    }
    
    type UserInfoResult struct {
        Result *UserInfo `json:result,omitempty`
        Error *Error `json:error,omitempty`
    }

But `UserInfoResults` is fine, apart from the lack of marshalling annotations.