ScreenScraper cookie support

[jantman]

Support for loading and saving cookies from a cookie file is broken in PhantomJS 2.x (see ariya/phantomjs#13115 ). We need a workaround for this.

• fix the ScreenScraper example in the docs to include actually getting a page - right now it's just doing Ajax stuff
• instead of self.browser.get(), use a self.get_page() to allow some wrapping logic
• make sure we're not calling browser close anywhere, or if we are, it's wrapped
• every time we get a page, if the browser already had a page loaded, call get_cookies() and save them to disk.
• When we first create the browser object, read the saved cookies from disk (if they exist) and pass them to the browser via add_cookie().