Add deleted property to resources that can be deleted

[snewcomer]

Under the assumption that when retrieving a deleted resource, that it will return the object with the deleted key. This is only mentioned in the custom docs however.

https://stripe.com/docs/api/customers/retrieve

closes #417

[coveralls]

Coverage remained the same at 82.505% when pulling ffc479e on snewcomer:sn/add-deleted-property into c15350a on code-corps:master.

[coveralls]

Coverage remained the same at 82.505% when pulling ffc479e on snewcomer:sn/add-deleted-property into c15350a on code-corps:master.

[snewcomer]

@praveenperera I'll wait a few days for your review before merging!

[praveenperera]

Looks good and I tested it works as expected:

{:ok,
 %Stripe.Customer{
   account_balance: nil,
   created: nil,
   currency: nil,
   default_source: nil,
   deleted: true,
   delinquent: nil,
   description: nil,
   discount: nil,
   email: nil,
   id: "cus_DleOHixcCyFTOW",
   invoice_prefix: nil,
   livemode: nil,
   metadata: nil,
   object: "customer",
   shipping: nil,
   sources: nil,
   subscriptions: nil,
   tax_info: nil,
   tax_info_verification: nil
 }}

Thanks! @snewcomer