WIP: Sidekiq Support

This is provides basic sidekiq support.

Currently functionality:

*  Automatically retry jobs in case of failure: this is handy in case mailchimp decides to have a bad day or whatever
*  Subscribing users
*  Subscribing subscribers

Currently NOT supported:

*  Unsubscribing user on destroy

Although I doubt this will happen a lot, it is currently broken. When you destroy a user, he does not get unsubscribed. This is because sidekiq is not able to find the object by ID. The object has already been destroyed. Trying to serialize the object seems to give a weird error atm. Still investigating.