New class for rendering CSS code inside a <style> tag.

Class CssRule allows to create and include css rules similarly to the generating html markup via Renderer class.
Two new external methods added:
Style::rule() - for initializing an instance of CssRule (but since constructor is private, it may be invoked directly via new).
Style::addRule($rule) - for inserting contents of an object into the <style> tag.

Almost all the methods of a new class are shown in Readme in examples 4 and 5.

No validation is used in order to a) give users max freedom b) exclude possibility of making some tricky or new features of css unachievable.