Add ability to create subscriptions without fetching customer record

This commit adds `Customer#create_subscription`, which allows a
subscription to be created on a customer without first fetching the
customer record.

Previously:
```ruby
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`:
```ruby
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.

Reference: #120

/cc @jimdanz