Pull subscription processing out of mailchimp_subscribe() and into mailchimp_subscribe_process().

As part of the async handling refactor, provide a function to directly
interact with the API.
Pull subscription processing out of mailchimp_update() and into mailchimp_update_process().
Cron no longer needs to pass the async flag.
Change mailchimp_update_process() to mailchimp_update_member_process() for consistency.
Refactor mailchimp_unsubscribe() to match the new async pattern.
Merge pull request #58 from thinkshout/async-refactor

Make cron async handling more robust and minimize duplicate code