Expose credit card verifications when present

When a customer is created with the verify_card flag, the results of
credit card verification are included in the XML response, nested within
the credit card.

This change exposes the verification data, which was previously being
discarded.

The verification data is useful for apps that may wish to, for example,
store the AVS and CVV results for fraud analysis and chargeback response
without having to separately look up those values from the Braintree
dashboard.

**Regarding tests** 

There did not seem to be unit specs for other nested attributes such
as CreditCard#subscriptions, so I did not add any unit spec for this
change. I am happy to do so if someone points me in the right direction.

***Example request/response***
```xml
[10/Jun/2014 21:34:49 UTC] PUT /customers/65888381
<?xml version="1.0" encoding="UTF-8"?>
<customer>
  <first-name>asdf</first-name>
  <last-name>asdf</last-name>
  <email>adsfasdfjkljl@kljklajsdf.com</email>
  <company>asdf</company>
  <phone>425-555-1212</phone>
  <credit-card>
    <number>411111******1111</number>
    <expiration-month type="integer">11</expiration-month>
    <expiration-year type="integer">2023</expiration-year>
    <cvv>***</cvv>
    <cardholder-name>asdf asdf</cardholder-name>
    <billing-address>
      <street-address>asdf</street-address>
      <extended-address nil="true"/>
      <locality>Redmond</locality>
      <region>WA</region>
      <postal-code>98052</postal-code>
    </billing-address>
    <options>
      <verify-card type="boolean">true</verify-card>
      <make-default type="boolean">true</make-default>
    </options>
    <device-data nil="true"/>
  </credit-card>
</customer>

[10/Jun/2014 21:34:49 UTC] PUT /customers/65888381 200
[10/Jun/2014 21:34:49 UTC] 200 OK
<?xml version="1.0" encoding="UTF-8"?>
<customer>
  <id>65888381</id>
  <merchant-id>ddvd4vfqq4fq25hr</merchant-id>
  <first-name>asdf</first-name>
  <last-name>asdf</last-name>
  <company>asdf</company>
  <email>adsfasdfjkljl@kljklajsdf.com</email>
  <phone>425-555-1212</phone>
  <fax nil="true"/>
  <website nil="true"/>
  <created-at type="datetime">2014-05-29T18:03:04Z</created-at>
  <updated-at type="datetime">2014-06-10T21:34:49Z</updated-at>
  <custom-fields>
  </custom-fields>
  <credit-cards type="array">
    <credit-card>
      <billing-address>
        <id>kf</id>
        <customer-id>65888381</customer-id>
        <first-name nil="true"/>
        <last-name nil="true"/>
        <company nil="true"/>
        <street-address>asdf</street-address>
        <extended-address nil="true"/>
        <locality>Redmond</locality>
        <region>WA</region>
        <postal-code>98052</postal-code>
        <country-code-alpha2 nil="true"/>
        <country-code-alpha3 nil="true"/>
        <country-code-numeric nil="true"/>
        <country-name nil="true"/>
        <created-at type="datetime">2014-06-10T21:34:49Z</created-at>
        <updated-at type="datetime">2014-06-10T21:34:49Z</updated-at>
      </billing-address>
      <bin>411111</bin>
      <card-type>Visa</card-type>
      <cardholder-name>asdf asdf</cardholder-name>
      <commercial>Unknown</commercial>
      <country-of-issuance>Unknown</country-of-issuance>
      <created-at type="datetime">2014-06-10T21:34:49Z</created-at>
      <customer-id>65888381</customer-id>
      <customer-location>US</customer-location>
      <debit>Unknown</debit>
      <default type="boolean">true</default>
      <durbin-regulated>Unknown</durbin-regulated>
      <expiration-month>11</expiration-month>
      <expiration-year>2023</expiration-year>
      <expired type="boolean">false</expired>
      <healthcare>Unknown</healthcare>
      <image-url>https://assets.braintreegateway.com/payment_method_logo/visa.png?environment=sandbox&amp;merchant_id=ddvd4vfqq4fq25hr</image-url>
      <issuing-bank>Unknown</issuing-bank>
      <last-4>1111</last-4>
      <payroll>Unknown</payroll>
      <prepaid>Unknown</prepaid>
      <subscriptions type="array"/>
      <token>52yrxg</token>
      <unique-number-identifier>5e411b2769fb040245fac898962f685e</unique-number-identifier>
      <updated-at type="datetime">2014-06-10T21:34:49Z</updated-at>
      <venmo-sdk type="boolean">false</venmo-sdk>
<!-- BEGINNING OF CURRENTLY DISCARDED VERIFICATION DATA -->
      <verifications type="array">
        <verification>
          <status>verified</status>
          <cvv-response-code>M</cvv-response-code>
          <avs-error-response-code nil="true"/>
          <avs-postal-code-response-code>M</avs-postal-code-response-code>
          <avs-street-address-response-code>M</avs-street-address-response-code>
          <gateway-rejection-reason nil="true"/>
          <merchant-account-id>864nkjcdvmsmgw57</merchant-account-id>
          <processor-response-code>1000</processor-response-code>
          <processor-response-text>Approved</processor-response-text>
          <id>hk3jvm</id>
          <billing>
            <first-name nil="true"/>
            <last-name nil="true"/>
            <company nil="true"/>
            <street-address>asdf</street-address>
            <extended-address nil="true"/>
            <locality>Redmond</locality>
            <region>WA</region>
            <postal-code>98052</postal-code>
            <country-name nil="true"/>
          </billing>
          <credit-card>
            <token>52yrxg</token>
            <bin>411111</bin>
            <last-4>1111</last-4>
            <card-type>Visa</card-type>
            <expiration-month>11</expiration-month>
            <expiration-year>2023</expiration-year>
            <customer-location>US</customer-location>
            <cardholder-name>asdf asdf</cardholder-name>
            <prepaid>Unknown</prepaid>
            <healthcare>Unknown</healthcare>
            <debit>Unknown</debit>
            <durbin-regulated>Unknown</durbin-regulated>
            <commercial>Unknown</commercial>
            <payroll>Unknown</payroll>
            <issuing-bank>Unknown</issuing-bank>
            <country-of-issuance>Unknown</country-of-issuance>
            <product-id>Unknown</product-id>
          </credit-card>
          <created-at type="datetime">2014-06-10T21:34:48Z</created-at>
          <updated-at type="datetime">2014-06-10T21:34:49Z</updated-at>
        </verification>
      </verifications>
<!-- END OF CURRENTLY DISCARDED VERIFICATION DATA -->
    </credit-card>
  </credit-cards>
  <addresses type="array">
    <address>
      <id>kf</id>
      <customer-id>65888381</customer-id>
      <first-name nil="true"/>
      <last-name nil="true"/>
      <company nil="true"/>
      <street-address>asdf</street-address>
      <extended-address nil="true"/>
      <locality>Redmond</locality>
      <region>WA</region>
      <postal-code>98052</postal-code>
      <country-code-alpha2 nil="true"/>
      <country-code-alpha3 nil="true"/>
      <country-code-numeric nil="true"/>
      <country-name nil="true"/>
      <created-at type="datetime">2014-06-10T21:34:49Z</created-at>
      <updated-at type="datetime">2014-06-10T21:34:49Z</updated-at>
    </address>
  </addresses>
</customer>
```