Setting Default Shipping Rates for Flat Rate Shipping

Some users that activate the Flat Rate Shipping or the International Flat Rate Shipping method accidentally forget to add a fee and then they see a confusing error message (`there are no shipping options available for your country`). Since the user clearly wants to use these methods we should set a default fee of `0`.

Then it will show up like this:

![default shipping fees](https://f.cloud.github.com/assets/1065372/963014/1f07fff0-04fa-11e3-9709-802d38f75527.png)

This is better than seeing a confusing error message and having no way to checkout.

Reference: https://woothemes.zendesk.com/agent/#/tickets/87178