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 what I mean: https://github.com/Behat/Mink/blob/master/src/Behat/Mink/Element/TraversableElement.php#L33

The `findById` method is in `TraversableElement` so the `findByName` should be.