FLEX-613 add_portal_cart_listings Contact model method

add listings to a portal cart
Expanding a tad on [my earlier comment](https://github.com/sparkapi/spark_api/pull/72#issuecomment-47661367), we'd prefer to keep cross-model convenience methods like this out of the models themselves. A cleaner implementation of this would be delegate creating a new cart to the Enumerable#find method in the application code. I'd envision something like this:

```ruby
new_cart = Proc.new { ListingCart.new(:contact_id => '1234', :ListingIds => [], :name => @cart_name) }
cart = contact.listing_carts.find(new_cart) {|cart| cart.Name == @cart_name}

cart.ListingIds << @listing_ids
cart.save
```

Does that make sense, even though it's a fairly contrived example? I'm reluctant to start adding convenience methods in here given how well separated the models are right now, and the potential for a lot of bloat if we were to start down this path. This feels more application-specific than something dealing with CRUD, which is the main intent of the gem.