More methods for NodeElement children manipulations

Added useful methods to get elements:

* getAllChildren()
* getDirectChildren()
* getFirstChild()
* getLastChild()
* getPreviousSiblings()
* getPreviousSibling()
* getNextSiblings()
* getNextSibling()
* findByName($name)
* findByTag($tag)
Ok, got you now.
But, "name" is not in the NamedSelector.
we can call as $this->find('named', array('id', $id));
or $this->find('named', array('button', $locator));
or $this->find('named', array('link', $locator));
But I haven't found: $this->find('named', array('name', $name));