Changed spree_mailchimp_gibbon to work with Spree 2.1 and use MailChimp API 2.0

Hello,

I recently upgraded a Spree shop and needed this gem to work, because the client had it on the 1.1 version too.

After some refactoring I was able to use it on my side. I've also created a '2-1-stable' branch in order to respect the terminology used on the spree/spree repo.

Hope this will be useful for somebody else.

Have a nice day/night.