安装三大浏览器驱动driver
1.chromedriver 下载地址:https://code.google.com/p/chromedriver/downloads/list
2.Firefox的驱动geckodriver 下载地址:https://github.com/mozilla/geckodriver/releases/
3.IE的驱动IEdriver 下载地址:http://www.nuget.org/packages/Selenium.WebDriver.IEDriver/