Improve Product CSV Import, admin panel ui layout and behaviour.

![Google ChromeScreenSnapz003](https://f.cloud.github.com/assets/1079437/384343/e408041c-a64a-11e2-929e-832fd968b394.png)
![Google ChromeScreenSnapz004](https://f.cloud.github.com/assets/1079437/384342/e3c832d8-a64a-11e2-9f8d-450232779a8b.png)

Screenshots: http://imgur.com/a/bJR4L

* present much documentation and clarify much import behaviour
* add support for more weight units. 
* change numbers to Excel style letters.
* show up to 5 rows of sample data. 
* support adding newly created products to no category.
* feedback how many records were imported. 

There are new and updated i18n strings in this pull request.