Updated available archives and method for pushing to some archives.

[a-mabe]

• Pushing to Archive Today and Megalodon now involves Selenium in order to avoid Captcha.
• Removed WebCite since they are no longer accepting archiving requests.
• Removed Archive.st since the Captcha is unavoidable, even with Selenium.
• Updated README to reflect these changes and explain how to properly setup Selenium in order to push to Archive Today and Megalodon from archivenow.