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.session do |client|
       client.http.read_timeout = 5
       response = client.api_request(...)
       # ...
     end
```

This fixes #79. /cc @alloy @rngtng @deepakl