Multiple sites

This pull allows for multiple sites to be checked.

Changes:
- There is a new tuple in the configuration file with all the sites to check.

        sites = (
            '8.8.8.8',
            '8.8.4.4',
        )

- The code has been split into classes for easier readability and so that you can import it in the Web UI

It works through a new file `down_sites_list.txt` which stores all the sites that were down in the last check.