Add Create Function to Invoice Object to Fit Documentation 

According to https://stripe.com/docs/api?lang=php#create_invoice the method Stripe_Invoice::create() should work. However, on attempting to run this the user gets a fatal error - undefined method. This change adds the create method back into the invoice object. As far as I can tell the change works perfectly. Alternately, if there is a valid reason why this method should not exist, can you update the documentation to reflect this?