More methods for NodeElement children manipulations

Added useful methods to get elements:

* getAllChildren()
* getDirectChildren()
* getFirstChild()
* getLastChild()
* getPreviousSiblings()
* getPreviousSibling()
* getNextSiblings()
* getNextSibling()
* findByName($name)
* findByTag($tag)
Now it's odd. I still don't get what you mean.

Have you understood my original comment about `NamedSelector` usage?