REST API client improvements

Some refactoring of the new REST API client.

You can now access the `Net::HTTP` instance by calling `client.http` so you can set custom options like `read_timeout`. This will only work before the client is used for any API request.

``` ruby
Adyen::REST.client.http.read_timeout = 1.0
```

This fixes #79. /cc @alloy @rngtng @deepakl