[*] FO : Improve product.js attributes/combination selection

A few changes to the way event listening was handled on attributes : 

Events listeners for combinations
---------------------
Removed the DOM0 listeners from the template, and switched to Jquery "on" event handler instead.

Explicit classes for JS purpose
-----------------------------------------
Prefixing classes which are required by JS code makes it easier for everyone to work with the templates. Here are the classes I added : 
```
.js-attributes // The container for all attributes selectors
.js-attribute_select // Attribute <select> tag
.js-attribute_color // Color picker
.js-attribute_color_hidden // The hidden color picker field
.js-attribute_radio // Atrribute <input type="radio" /> tag
```

Less DOM traversing
----------------------------
```$(elt).parent().parent().parent().children('.color_pick_hidden,#color_pick_hidden')``` and similar selectors break easily if any changes are made to the HTML code. I cleaned up some of them.

If you are OK with those changes, I'll be more than happy to push further on simplifying product.js !
Regards,