Direct access to more low level driver methods through magic method

All methods with WebDriver\Session are now available through the Selenium2Driver. Methods with Behat\Mink\Driver\Selenium2Driver have been overwritten.

```
__call                    Behat\Mink\Driver\Selenium2Driver
__construct               Behat\Mink\Driver\Selenium2Driver
__toString                WebDriver\Session
getURL                    WebDriver\Session
element                   WebDriver\Session
elements                  WebDriver\Session
locate                    WebDriver\Session
accept_alert              WebDriver\Session
activeElement             WebDriver\Session
alert_text                WebDriver\Session
application_cache         WebDriver\Session
back                      Behat\Mink\Driver\Selenium2Driver
browser_connection        WebDriver\Session
buttondown                WebDriver\Session
buttonup                  WebDriver\Session
capabilities              WebDriver\Session
click                     Behat\Mink\Driver\Selenium2Driver
close                     WebDriver\Session
cookie                    WebDriver\Session
deleteAllCookies          WebDriver\Session
deleteCookie              WebDriver\Session
deleteWindow              WebDriver\Session
dismiss_alert             WebDriver\Session
doubleclick               WebDriver\Session
execute                   WebDriver\Session
execute_async             WebDriver\Session
execute_sql               WebDriver\Session
focusWindow               WebDriver\Session
forward                   Behat\Mink\Driver\Selenium2Driver
frame                     WebDriver\Session
getAllCookies             WebDriver\Session
ime                       WebDriver\Session
keys                      WebDriver\Session
local_storage             WebDriver\Session
location                  WebDriver\Session
moveto                    WebDriver\Session
open                      WebDriver\Session
orientation               WebDriver\Session
refresh                   WebDriver\Session
screenshot                WebDriver\Session
session_storage           WebDriver\Session
setCookie                 Behat\Mink\Driver\Selenium2Driver
source                    WebDriver\Session
timeouts                  WebDriver\Session
title                     WebDriver\Session
touch                     WebDriver\Session
url                       WebDriver\Session
window                    WebDriver\Session
window_handle             WebDriver\Session
window_handles            WebDriver\Session
```

Methods that were already available:
```
attachFile                Behat\Mink\Driver\Selenium2Driver
blur                      Behat\Mink\Driver\Selenium2Driver
check                     Behat\Mink\Driver\Selenium2Driver
doubleClick               Behat\Mink\Driver\Selenium2Driver
dragTo                    Behat\Mink\Driver\Selenium2Driver
evaluateScript            Behat\Mink\Driver\Selenium2Driver
executeScript             Behat\Mink\Driver\Selenium2Driver
find                      Behat\Mink\Driver\Selenium2Driver
focus                     Behat\Mink\Driver\Selenium2Driver
getAttribute              Behat\Mink\Driver\Selenium2Driver
getContent                Behat\Mink\Driver\Selenium2Driver
getCookie                 Behat\Mink\Driver\Selenium2Driver
getCurrentUrl             Behat\Mink\Driver\Selenium2Driver
getHtml                   Behat\Mink\Driver\Selenium2Driver
getResponseHeaders        Behat\Mink\Driver\Selenium2Driver
getScreenshot             Behat\Mink\Driver\Selenium2Driver
getStatusCode             Behat\Mink\Driver\Selenium2Driver
getTagName                Behat\Mink\Driver\Selenium2Driver
getText                   Behat\Mink\Driver\Selenium2Driver
getValue                  Behat\Mink\Driver\Selenium2Driver
isChecked                 Behat\Mink\Driver\Selenium2Driver
isStarted                 Behat\Mink\Driver\Selenium2Driver
isVisible                 Behat\Mink\Driver\Selenium2Driver
keyDown                   Behat\Mink\Driver\Selenium2Driver
keyPress                  Behat\Mink\Driver\Selenium2Driver
keyUp                     Behat\Mink\Driver\Selenium2Driver
mouseOver                 Behat\Mink\Driver\Selenium2Driver
reload                    Behat\Mink\Driver\Selenium2Driver
reset                     Behat\Mink\Driver\Selenium2Driver
resizeWindow              Behat\Mink\Driver\Selenium2Driver
rightClick                Behat\Mink\Driver\Selenium2Driver
selectOption              Behat\Mink\Driver\Selenium2Driver
setBasicAuth              Behat\Mink\Driver\Selenium2Driver
setRequestHeader          Behat\Mink\Driver\Selenium2Driver
setSession                Behat\Mink\Driver\Selenium2Driver
setValue                  Behat\Mink\Driver\Selenium2Driver
start                     Behat\Mink\Driver\Selenium2Driver
stop                      Behat\Mink\Driver\Selenium2Driver
switchToIFrame            Behat\Mink\Driver\Selenium2Driver
switchToWindow            Behat\Mink\Driver\Selenium2Driver
uncheck                   Behat\Mink\Driver\Selenium2Driver
visit                     Behat\Mink\Driver\Selenium2Driver
wait                      Behat\Mink\Driver\Selenium2Driver
getDefaultCapabilities    Behat\Mink\Driver\Selenium2Driver
getWebDriverSession       Behat\Mink\Driver\Selenium2Driver
setDesiredCapabilities    Behat\Mink\Driver\Selenium2Driver
setWebDriver              Behat\Mink\Driver\Selenium2Driver
```