Add static handling to mailchimp_get_api_object().
Spelling.
Cron no longer needs to pass mcapi object as a parameter.

Since mailchimp_get_api_object() is now static, we can safely call it
each time.
Remote `$mcapi = NULL` parameter from subscribe functions.

Removed from `mailchimp_subscribe()`, `mailchimp_update_member()` and
`mailchimp_unsubscribe()`. Any case that would need to pass in the
parameter can now rely on it being statically cached.