Add methods and examples for offline testing

Add methods and examples for offline testing:

• Add methods for injecting html into a web page

self.load_html_string(html_string, new_page=True)
self.load_html_file(html_file, new_page=True)
self.open_html_file(html_file)

• Use complete cloudflare links for accessing libraries
• Add assert_raises() to avoid the camel-casing of assertRaises()
• Update the beautifulsoup and certifi dependences
• Improve highlighting in Demo Mode
• Increase the bring_to_front() z-index setting
• Make sure jQuery is fully activated before using it
• Add an offline example with a single-page web app