Use list names instead of ids to ease customizations

By making use of mailchimp names instead of ids it's possible to code-customizations for special mailchimp lists without having to refer to an possible changing id - thus that way customizations are much more stable.

Attached patch fixes the list module to use the machine name for block deltas, form ids and inside $form. What allows stuff like
* configure a mailchimp block via context module and export that
* theme blocks by name using the block template suggestions (module, delta)
* theme a signup form specifically for a mailchimp list by name
* alter signup forms by name
...

I think that should cover the most important places where it's very useful to have names.