Clean up Token, BankAccount, Card, Source types

[joshsmith]

Note that this adds a token_bank_account and token_card as the OpenAPI specs for Stripe indicate that these have different schemas from our own Stripe.BankAccount and Stripe.Card.

I've also now added Stripe.Types.metadata so we can stop repeating that key.

Also moved some of the types like account_holder_type, status, tokenization_method, etc. into the relevant entities such as Stripe.BankAccount.status.

This also expands on the Source types and their inner structure.

[coveralls]

Coverage remained the same at 55.587% when pulling 8d9a78c on clean-up-token-types into c803f92 on 2.0-beta.