Add ItemSearchResponse class

Currently Vacuum seems to be way too lightweight, basically you have to implement all response parsing by yourself. This PR adds ItemSearchResponse class so that common tasks can be handler easier.

Usage example
```ruby
req = Vacuum.new
result = Vacuum::ItemSearchResponse.new(req.item_search(query: params))
if result.is_valid?
  puts result.items.to_a.first.item_attributes['Title']
else
  e = result.error
  raise "#{e['Code']}:#{e['Message']}"
end
```

Rename `is_valid?` to `valid?`.