Unicode window title support.

Following https://github.com/LaurentGomila/SFML/pull/351, sf::Window now expects an sf::String to be passed to sf::Window::Window and sf::Window::setTitle. This patch adds a wrapper to convert a Python string to an sf::String and pass the appropriate strings to before-mentioned functions.

This patch also fixes an error where passing no title to RenderWindow would lead to an AttributeError.