Adds support for custom fields when adding a subscriber

The API for submitting new subscribers isn't ideal but this adds some support for submitting custom fields.

```ruby
api = Interspire::API.new('http://iem.atechmedia.com/xml.php', 'user', 'pass')
api.add_subscriber('test@test.com', true, 'html', {1 => 'Bananas', 2 => 'Apples'})
```