More methods for NodeElement children manipulations

Added useful methods to get elements:

* getAllChildren()
* getDirectChildren()
* getFirstChild()
* getLastChild()
* getPreviousSiblings()
* getPreviousSibling()
* getNextSiblings()
* getNextSibling()
* findByName($name)
* findByTag($tag)
Of course it's not there. I'm telling that you should add it there and then use it.