Added small method called findfirst(f, seq) (added to djstripe.models for now), which returns first item in sequence where f(item) == True.

In djstripe.models.Customer.update_card I call findfirst with f=(lambda card: card['id'] == Customer.stripe_customer.default_card) and seq=Customer.stripe_customer.cards.data

[1b549d5] Made use of the card returned by findfirst
[1ef1c62] typo fix
[ada2681] models and utils would import each other, so i moved the function to models for now
[ae2c5c9] First test