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

Hello,

I've created a new branch from the master and did all the refactoring again.

I've also updated the README.textile according to the new fixes. I also added in there that maybe there should be a '2-1-stable' branch on which this code might be added, in order to respect the branch terminologies from spree/spree and other plugins for Spree:

```ruby
gem 'spree_mailchimp_gibbon', github: 'bluehandtalking/spree_mailchimp_gibbon', branch: '2-1-stable'
```