Basic pagination

Add basic pagination based on `NextPageLink` parameter returned via API. It is accessible via `next_page` and `next_page?` methods. Example usage:

```ruby
first_page = api_client.contact.all
second_page = api_client.next_page if api_client.next_page?
```
Also add method for fetching only one model from myob api:

```ruby
contact = api_client.contact.find(contact_uid)
```
This should be `api_client.contact.next_page` - I'll fix that as part of the merge