More methods for NodeElement children manipulations

Added useful methods to get elements:

* getAllChildren()
* getDirectChildren()
* getFirstChild()
* getLastChild()
* getPreviousSiblings()
* getPreviousSibling()
* getNextSiblings()
* getNextSibling()
* findByName($name)
* findByTag($tag)
this is wrong. It always returns a list of NodeElement, even when being called on a DocumentElement instance. It should be ``NodeElement[]`` (it cannot be ``null`` either)