More methods for NodeElement children manipulations

Added useful methods to get elements:

* getAllChildren()
* getDirectChildren()
* getFirstChild()
* getLastChild()
* getPreviousSiblings()
* getPreviousSibling()
* getNextSiblings()
* getNextSibling()
* findByName($name)
* findByTag($tag)
I don't like this method much. The name is not a unique identifier at all, so I'm not sure returning a single element makes sense