Fix parsing GetProductCategoriesForASIN response

The amazon mws GetProductCategoriesForASIN api call returns multiple <Self> nodes (if available), but the boto code returned only the last XML node.

This change will break some people's code if they are assuming that there is only one Self node, but it is the right fix.

API Response (from the integration test):

```xml
<?xml version="1.0" encoding="UTF-8"?>
<GetProductCategoriesForASINResponse xmlns="http://mws.amazonservices.com/schema/Products/2011-10-01">
   <GetProductCategoriesForASINResult>
      <Self>
         <ProductCategoryId>285856</ProductCategoryId>
         <ProductCategoryName>Python</ProductCategoryName>
         <Parent>
            <ProductCategoryId>3952</ProductCategoryId>
            <ProductCategoryName>Languages &amp; Tools</ProductCategoryName>
            <Parent>
               <ProductCategoryId>3839</ProductCategoryId>
               <ProductCategoryName>Programming</ProductCategoryName>
               <Parent>
                  <ProductCategoryId>5</ProductCategoryId>
                  <ProductCategoryName>Computers &amp; Technology</ProductCategoryName>
                  <Parent>
                     <ProductCategoryId>1000</ProductCategoryId>
                     <ProductCategoryName>Subjects</ProductCategoryName>
                     <Parent>
                        <ProductCategoryId>283155</ProductCategoryId>
                        <ProductCategoryName>Subjects</ProductCategoryName>
                     </Parent>
                  </Parent>
               </Parent>
            </Parent>
         </Parent>
      </Self>
      <Self>
         <ProductCategoryId>21</ProductCategoryId>
         <ProductCategoryName>Education &amp; Reference</ProductCategoryName>
         <Parent>
            <ProductCategoryId>1000</ProductCategoryId>
            <ProductCategoryName>Subjects</ProductCategoryName>
            <Parent>
               <ProductCategoryId>283155</ProductCategoryId>
               <ProductCategoryName>Subjects</ProductCategoryName>
            </Parent>
         </Parent>
      </Self>
   </GetProductCategoriesForASINResult>
   <ResponseMetadata>
      <RequestId>DUMMY REQUEST ID</RequestId>
   </ResponseMetadata>
</GetProductCategoriesForASINResponse>
```