Changed RedirectURLs to pointer from slice

[nazwa]

Redirect urls are kept in a single struct, not a slice.

[nazwa]

This is what comes back from a request with a slice:

2015/03/20 10:30:06 POST https://api.sandbox.paypal.com/v1/payments/payment: 400 Incoming JSON request does not map to API request

And response with a pointer:

{
    "intent": "sale",
    "payer": {
        "payment_method": "paypal",
        "payer_info": {
            "shipping_address": {
                "line1": "",
                "city": "",
                "country_code": ""
            }
        }
    },
    "transactions": [
        {
            "amount": {
                "currency": "GBP",
                "total": "10.00",
                "details": {
                    "subtotal": "10.00"
                }
            },
            "description": "Points"
        }
    ],
    "id": "PAY-5GS05962PW1226719KUF6WKA",
    "create_time": "2015-03-20T09:40:56Z",
    "state": "created",
    "update_time": "2015-03-20T09:40:56Z",
    "links": [
        {
            "href": "https://api.sandbox.paypal.com/v1/payments/payment/PAY-5GS05962PW1226719KUF6WKA",
            "rel": "self",
            "method": "GET",
            "enctype": ""
        },
        {
            "href": "https://www.sandbox.paypal.com/cgi-bin/webscr?cmd=_express-checkout&token=EC-8J9661131N9664043",
            "rel": "approval_url",
            "method": "REDIRECT",
            "enctype": ""
        },
        {
            "href": "https://api.sandbox.paypal.com/v1/payments/payment/PAY-5GS05962PW1226719KUF6WKA/execute",
            "rel": "execute",
            "method": "POST",
            "enctype": ""
        }
    ]
}