Support for Bank Account Tokenization (Issue #44)

Hey guys, over here at Zola we needed to add support for bank account tokenization in iOS. This adds that functionality, as described in #44.

This PR includes:
- `STPBankAccount` model object
-  `createTokenWithBankAccount:` methods added to `Stripe.h`
- `STPBankAccount` unit tests in `STPBankAccountTest`
- Functional tests for bank account tokenization in `STPBankAccountFunctionalTest`

I couldn't find the human-readable implementations for `validateRoutingNumber` and `validateAccountNumber` by country (as is included in `Stripe.js`), so I opted not to include these methods since I'm not sure how many countries you support. It'd be great if you could provide those methods.