Made "join_mailing_list" attribute persistant
Fixed a bug with contact retrieval.

As email is a key of the contact resource, I must use Mailjet::Contact.find(email) instead of Mailjet::Contact.first(email: email)

+ Optimisations : never resubscribe an already subscribed contact and never unsubscribe an already unsubscribed contact.
+ Refactoring: pluralized the LISTS_CACHE_KEY and CONTACTS_CACHE_KEY constants
Bump to version 0.0.3 "It may work now"