This commit adds `Customer#create_subscription`, which allows a
subscription to be created on a customer without first fetching the
customer record.
Previously:
```
customer = Stripe::Customer.retrieve('cus_abc123def') # GET request
customer.subscriptions.create(plan: 'cool-plan-1') # POST request
```
**No alteration has been made to the above method; the preceding
implementation still functions as it did previously.**
With `#create_subscription`:
```
customer = Stripe::Customer.new('cus_abc123def') # No request
customer.create_subscription(plan: 'cool-plan-1') # POST request
```
This method removes the initial `GET` request and instead issues a
`POST` directly to create the subscription.
It seems to embrace the 'Ruby Way' (and more convenient in my own code) to be able to access an upcoming invoice from the customer itself rather than de-reference the customer id and ask the Invoice class itself for that info.
