Add ability to add Constant Contact lists as user entity field

Ok here's a patch to test out. It may need some work but basically I have a field type that pulls in lists. Then you can subscribe/unsubscribe on entity save and/or delete. Let me know what you think!

Merged this into 4.x-dev release →

Rolled into 4.0.0-rc1

Updated documentation.
Adjusted the field configuration a bit.
I think this may be ready for release.

Decided I should use 3.1.x instead of 4. This isn't really a breaking change.

@rosemarystanley Wow, thanks so much! I'll review this out ASAP!

Fixed typo on 'stand-alone'.

Marking as fixed. We can reopen or create new issue if there is more to fix.