Create credit cards for customers through TR
Allow for calling Braintree::Customer.create with no params

This is legal but fake_braintree chokes on it
Add some (very) rudimentary validation support
Refactor confusing 'more_opts' line in create_payment_method

This code was complicated because I was trying to handle the case where someone posts to
create_payment_method without a 'credit_card' param. But that should be an error
anyway, so why handle it?
Style fixes per pull request
Make hash_from_request_body_with_key always return a hash

It was returning a blank string in the case where the key had no value. Changed
to return an empty hash.
Allow Braintree::CreditCard.create without customer

This is valid, according to the docs.
Use my version of credit card creation

IMO the version in master is a bit smelly - you'd think this would belong in
credit_card.rb. This version also remembers to update the customer hash when
creating a credit card for a customer. And lastly, this runs a check against the
decline_all_cards/verify_all_cards flags, which should be useful for testing
credit card TR.