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.
removed interfaces and added concreated for the lists for serialization
Added expand support to the included models, services have not been updated yet. This change should be transparent.
name change of StripeSubscriptions to StripeSubscriptionList on customer model
added metadata to command objects
Parameter builder was not including metadata due to its structure, updated to look for the field and properly create the query string.
updated to support subscription relationships
added null check for internal properties
Merge