Baixei .jar versão 3.1.0 do selenium server standalone, porém deu o seguinte erro ao executar o exemplo do curso de selenium:

import org.openqa.selenium.By;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.WebElement;
import org.openqa.selenium.firefox.FirefoxDriver;

public class TesteAutomatizado {

    public static void main(String[] args) {
        // abre firefox
        WebDriver driver = new FirefoxDriver();

        // acessa o site do google
        driver.get("http://www.google.com.br/");

        // digita no campo com nome "q" do google
        WebElement campoDeTexto = driver.findElement(By.name("q"));
        campoDeTexto.sendKeys("Caelum");

        // submete o form
        campoDeTexto.submit();

    }
}

Exception in thread "main" java.lang.IllegalStateException: The path to the driver executable must be set by the webdriver.gecko.driver system property; for more information, see https://github.com/mozilla/geckodriver. The latest version can be downloaded from https://github.com/mozilla/geckodriver/releases
    at com.google.common.base.Preconditions.checkState(Preconditions.java:738)
    at org.openqa.selenium.remote.service.DriverService.findExecutable(DriverService.java:111)
    at org.openqa.selenium.firefox.GeckoDriverService.access$100(GeckoDriverService.java:38)
    at org.openqa.selenium.firefox.GeckoDriverService$Builder.findDefaultExecutable(GeckoDriverService.java:112)
    at org.openqa.selenium.remote.service.DriverService$Builder.build(DriverService.java:302)
    at org.openqa.selenium.firefox.FirefoxDriver.createCommandExecutor(FirefoxDriver.java:312)
    at org.openqa.selenium.firefox.FirefoxDriver.<init>(FirefoxDriver.java:272)
    at org.openqa.selenium.firefox.FirefoxDriver.<init>(FirefoxDriver.java:267)
    at org.openqa.selenium.firefox.FirefoxDriver.<init>(FirefoxDriver.java:263)
    at org.openqa.selenium.firefox.FirefoxDriver.<init>(FirefoxDriver.java:122)
    at TesteAutomatizado.main(TesteAutomatizado.java:10)

O que devo fazer?