System.CalloutException: Unauthorized endpoint, please check Setup->Security->Remote site settings. endpoint = https://sandbox-goku.cs87.force.com/services/oauth2/token

public class Mycontroller {
        public String accessToken {
            private set;
            get {
                if (accessToken == null) {
                    accessToken = RestUtil.getAccessToken();
                }

                return accessToken;
            }
        }
  }

public class RestUtil {
    private transient static String accessToken = '';

    public static String getAccessToken() {
        if (accessToken == '') {
            accessToken = fetchAccessToken();
        }

        return accessToken;
    }

    private static String fetchAccessToken() {
        HttpRequest authReq = buildAutHttpRequest();

        Http http = new Http();

        HttpResponse authRes = http.send(authReq);

        String accessToken = extractAccessToken(authRes);

        return accessToken;
    }

    private static String extractAccessToken(HttpResponse authRes) {
        Map<String, Object> authResult = (Map<String, Object>) JSON.deserializeUntyped(authRes.getBody());

        return (String) authResult.get('access_token');
    }

    private static HttpRequest buildAutHttpRequest() {
        HttpRequest authReq = new HttpRequest();
        authReq.setEndpoint('https://' + URL.getSalesforceBaseUrl().getHost() + '/services/oauth2/token');
        authReq.setMethod('POST');
        authReq.setBody(buildAuthRequestBody());

        return authReq;
    }

    private static String buildAuthRequestBody() {
        String template = 'grant_type=password&username={0}&password={1}&client_id={2}&client_secret={3}';
        String[] arguments = new String[]{
                AdminAuthContants.USER_NAME,
                AdminAuthContants.PASSWORD,
                AdminAuthContants.CLIENT_ID,
                AdminAuthContants.CLIENT_SECRET
        };

        return String.format(template, arguments);
    }
}