More methods for NodeElement children manipulations

Added useful methods to get elements:

* getAllChildren()
* getDirectChildren()
* getFirstChild()
* getLastChild()
* getPreviousSiblings()
* getPreviousSibling()
* getNextSiblings()
* getNextSibling()
* findByName($name)
* findByTag($tag)
called this method  public function find($selector, $locator) in .../Mink/src/Behat/Mink/Element/Element.php
'xpath' is a selector, like 'css'.