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 get. Why you need to perform Xpath-safe escaping of the string if it is a CSS selector?

Also since this method doesn't do anything special and people can really give any CSS string instead tag, then it creates more confusion, than benefits it can provide.