[WIP] Improved products page (backend)

**DO NOT MERGE, FEEDBACK NEEDED**

Want to make the products page to show more useful information. Still trying to avoid adding page specific style if possible. (use purely Bootstrap)

Before:

![screen shot 2013-11-25 at 9 35 23 pm](https://f.cloud.github.com/assets/193112/1613325/791688d6-55d6-11e3-95ce-ff9b792c62f0.png)

After

![screen shot 2013-11-25 at 9 19 57 pm](https://f.cloud.github.com/assets/193112/1613307/3d98404c-55d6-11e3-88bb-dda9b44e14b6.png)

### Questions

1. Does master variant suppose to have stock on hand? Isn't it just a group?
2. What is the best way to add a new string? (`sylius.product.updated_at` for example)
3. Should we remove the edit and delete button? Should leave some space for users to add their own column (brand etc)
4. How do you want to change this page?