Fixes to allow handling of regular credit card transactions

[lfjeff]

I've been using the Braintree API directly and decided to migrate our credit card processing code to Omnipay.

I was surprised to discover that Omnipay/Braintree only supports transactions using a nonce and did not handle traditional processing where the card number is available.

This pull request fixes that problem. All the nonce stuff still works, but now it can also handle regular card transactions.