Suggestion: add a new method resetHostBase

This is a suggestion after struggling to reset the host base, that needs resetting after the use of amOnUrl()
http://codeception.com/docs/modules/WebDriver#amonurl

My suggestion is the following:
Although, it can be written much better probably

``` php
public function resetHostBase(){
$config = \Codeception\Configuration::config();
$url = $config["modules"]["config"]["WebDriver"]["url"];
$this->amOnUrl($url);
}
```

I believe that this function is needed in many cases after the use of amOnUrl() when using the WebDriver.

Thank you!