Lets not be too shy about just using `extend` instead of `include` here
when it's more appropriate to do so. The advantage to this approach is
that the module can be either extended _or_ included with this change,
but couldn't be without it due to the `ClassMethods` meta-magic.
List has already started doing this as of #314, so we don't have to be
afraid of breaking convention here.
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.
