Add bin, card_type, last_4, and unique_number_identifier to credit cards.
Support custom subscription IDs.

https://www.braintreepayments.com/docs/ruby/subscriptions/create\#subscription_id
Merge remote-tracking branch 'brandon-beacher/add_more_fields_to_credit_cards'

* brandon-beacher/add_more_fields_to_credit_cards:
  Add bin, card_type, last_4, and unique_number_identifier to credit cards.

Conflicts:
	lib/fake_braintree/credit_card.rb
Merge remote-tracking branch 'thoughtbot/master'

Conflicts:
	lib/fake_braintree/credit_card.rb
	spec/fake_braintree/credit_card_spec.rb