LOGIN

    public static boolean login(String username, String password) {
        String response = requestUrl(SERVER_URL, "username="+username+"&password="+password);

        if(response != null && response.startsWith("OK")){
            sessionID = response.substring(2);
            return true;
        }
        sessionID=null;
        return false;
    }

    public static String post(ArrayList<String> parametros){
        StringBuilder sb = new StringBuilder();
        if(sessionID == null){
            return null;
        }
        sb.append("sess_id=" + sessionID);
        for(int i=0;i<parametros.size();i++){
            sb.append("&" + parametros.get(i));
        }
        String response = requestUrl(SERVER_URL, sb.toString());
        Log.i("SERVER:",response);
        return response;
    }