Implement the standard gateway interfaces using the Stripe gem

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There is a number of standard methods that should be implemented on the payment gateway as described in this guide: https://guides.solidus.io/advanced-solidus/payments-and-refunds/#custom-payment-gateways.

The gateway is expected to implement these methods using the Stripe gem (and not ActiveMerchant):

• #initialize(options): initializes the gateway with the provided options. By default, Solidus will pass the payment method's preferences in here.
• #authorize(money, source, options = {}): authorizes a certain amount on the provided payment source.
• #capture(money, transaction_id, options = {}): captures a certain amount from a previously authorized transaction.
• #purchase(money, source, options = {}): authorizes and captures a certain amount on the provided payment source.
• #void(transaction_id, [source,] options = {}): voids a previously authorized transaction, releasing the funds that are on hold. The source parameter is only needed for payment gateways that support payment profiles.
• #credit(money, [source,] transaction_id, options = {}): refunds the provided amount on a previously captured transaction. The source parameter is only needed for payment gateways that support payment profiles.