Provide a configurable myshopify domain.

@peterjm @christhomson @EiNSTeiN-

myshopify domain now becomes an option when setting up the omniauth builder. Defaults to `myshopify.com`. 

I have also changed the valid_site? method to actually return a boolean in both cases and added a spec.

```
Rails.application.config.middleware.use OmniAuth::Builder do
  provider :shopify, ENV['SHOPIFY_API_KEY'], ENV['SHOPIFY_SHARED_SECRET'],
    myshopify_domain: 'myshopify.io'
end
```

I could rename the option, but it shouldn't be changed often.