Refactor Product fixture, introduce helpers

- Refactored Product fixture by extracting responsibility (_getWebsiteIds_ and such) and by introducing the concept of "helpers". A _Website_ helper has been created to help the Product fixture getting what's related to "_websites_".
- Adjusted ProductSpec accordingly.
- Commented completely CoreConfigSpec's pending spec causing PhpSpec to return non-zero value 
(there is a TODO in the code).

Hopefully with this we should be back to green :P
Assuming the usefulness of the helper(s), that method could be used by some other classes in the future by passing some parameters to the Mage's one.