More methods for NodeElement children manipulations

Added useful methods to get elements:

* getAllChildren()
* getDirectChildren()
* getFirstChild()
* getLastChild()
* getPreviousSiblings()
* getPreviousSibling()
* getNextSiblings()
* getNextSibling()
* findByName($name)
* findByTag($tag)
@aik099 please refer to     
public function getParent()
    {
        return $this->find('xpath', '..');
    }
in .../Mink/src/Behat/Mink/Element/NodeElement.php
I mean I can use the find method like find('xpath'...