Allow array params

This change allows one to setup array params in methods such as `find_items_advanced`, which would enable using `itemFilter` as described [here](http://developer.ebay.com/DevZone/finding/CallRef/findItemsAdvanced.html).

Here is a concrete example. I want to search for items in a given category, but I only want to see top rated sellers and sellers with a business type of `Business`. The URL would be (replace `app-id` with your app id):

http://svcs.ebay.com/services/search/FindingService/v1?OPERATION-NAME=findItemsAdvanced&SERVICE-VERSION=1.0.0&SECURITY-APPNAME=APP-ID&RESPONSE-DATA-FORMAT=JSON&RESTPAYLOAD=true&categoryId=267&itemFilter(0).name=SellerBusinessType&itemFilter(0).value=Business&itemFilter(1).name=TopRatedSellerOnly&itemFilter(1).value=true

Note `itemFilter(0)` and `itemFilter(1)`, which indicate an array of values. With the change I've introduced, one can do the following:

```ruby
response = finder.find_items_advanced({
  categoryId: 267,
  outputSelector: 'SellerInfo',
  itemFilter: [{
    name: 'TopRatedSellerOnly',
    value: 'true'
  },{
    name: 'SellerBusinessType',
    value: 'Business'
  }]
})
```

What do you think about this?