Add API changes to support the new MuliSubscription feature that is currently in beta. Marked some methods obsolete that are no longer going to operate as expected and noted their replacement. Added metadata fields to needed models and update the invoiceitems to allow relating to the specific subscription.

More details can be found here: https://gist.github.com/jimdanz/f318ab4feea6e023f480

I have not updated and tests or documentation as of yet but the code is there.