* Rename the `Update` operation to `Save`
* Add the `update` class method to all saveable resources
* Add tests for update method
* Add tests for plans, invoice items, and application fees
Usage on a top-level collection:
```
Stripe::Customer.list.auto_paging_each do |customer|
puts customer
end
```
Usage on a subcollection:
``` ruby
customer.invoices.auto_paging_each do |invoice|
puts invoice
end
```
We've also renamed `#all` to `#list` to prevent confusion ("all" implies
that all resources are being returned, and in Stripe's paginated API
this was not the case). An alias has been provided for backward API
compatibility.
Fixes#167.
Replaces #211 and #248.
* This error message said "Cards cannot be retrieved without a customer
ID. Retrieve a card using customer.cards.retrieve('card_id')"
This is incorrect. `customer.cards` throws a `NoMethodError: undefined
method `cards' for #<Stripe::Customer:0x0000010c8bdca8>`.
* The correct method is `Stripe::Customer#sources`, and this fixes the
error message.
