Using native Selenium2 command for `mouseOver`

This is backed up by a CSSDriverTest which has been added to Mink via PR https://github.com/Behat/Mink/pull/380

https://github.com/robsquires/MinkSelenium2Driver/commit/bda5090484ca78ddad451f6c9d9e5d487bcfa8b7 is necessary because as a result on this change, the mouseover event will not be triggered unless the cursor actually enters the focus of the element (hence moving it to the beginning of the sequence)