Problem with .co.uk and other two level registrar controlled domains.

The site tag generated can be weird when using .co.uk or other two level registrar controlled domains.

For example:
  * http://amazon.com/ -> amazon
  * http://amazon.co.uk/ -> co
  * http://www.amazon.co.uk/ -> amazon.co
  * http://www.aws.amazon.co.uk/ -> aws.amazon.co

I've prepared a commit which uses the tld.js library to deal with domains better:

  * http://amazon.com/ -> amazon
  * http://amazon.co.uk/ -> amazon
  * http://www.amazon.co.uk/ -> amazon
  * http://www.aws.amazon.co.uk/ -> amazon