Adds FetchXml Builder and response support.

```ruby
xml = %Q{<fetch version="1.0" output-format="xml-platform" mapping="logical" distinct="false">
  <entity name="opportunityproduct">
    <attribute name="opportunityproductid" />
    <attribute name="productid" />
    <attribute name="productdescription" />
    <attribute name="priceperunit" />
    <attribute name="quantity" />
    <order attribute="productid" descending="false" />
  </entity>
</fetch>}

result = client.fetch(xml)
# => #<DynamicsCRM::XML::EntityCollection>
# result.entity_name => 'opportunityproduct'
# result.entities => [DynamicsCRM::XML::Entity, ...]
```