Client side encryption

I've added ability to use Client Side Encryption with Adyen payments through Spree. This method uses aditional EncryptedCreditCard model which does not store any important data in the database. Probably it should be changed to not use database at all to be PCI compliant.

Please take a look at it and review it.

Thanks.

--
Sorry for the mess in commits: 'b1e885f' and '1600aec' are irrelevant - I messed with branching.