Paypal integration additions and updates to CCTransactionResult mapping

[erincdustin]

This PR includes the following additions to the PayPal integration:

• An optional isCapture bool argument for the InitializePaymentRequestAsync() method, which will flag the transaction intent as 'capture' is set to true (default 'authorize')
• A CapturePaymentAsync() method that captures funds immediately with no prior auth. This has been added to the ICreditCardProcessor interface but the method is not implemented in any other payment processor integrations at this time.
• New PayPalClient config properties: PartnerAttributionID and Merchants for further customization of the integration

And the following updates:

• The PayPal integration will now return a List of InnerTransactions for each merchant purchase_unit that is evaluated in a transaction. For example, the overall paypal capture can succeed, but it is possible for one or more merchant transactions to fail within the purchase_unit results.

[amrarick26]

This looks great!