PaymentService.getPaymentTypes and PaymentService.getCoinbaseAccounts fail due to various data type issues

[Duane May]

Call to GET /payment-methods fails due to:

• limits expecting an array, but the response is actually a single value
• the return type expected a single PaymentTypes object, but the response is actually an array

See https://docs.gdax.com/#payment-methods

This was fixed by #20 commit 500657d

Call to GET /coinbase-accounts fails due to:

• the return type expected a single CoinbaseAccounts object, but the response is actually an array

See https://docs.gdax.com/#coinbase-accounts

This was fixed by #20 commit f422a9e

[Ishmael Rufus]

Fixed in #24