Add products improvements

Beware! This is a master commit, so take it easy.

Main changes:
* New form to add products quickly and easily (see screenshot below)
* Now products can have 2 units in Warehouse A, and 1 units in Warehouse B. For this to work, I have added a new "product_warehouses" table
* The format of the barcode goes in application.rb. Make sure that you copy the example to your local environment.
* The values for the variations are not comma separated anymore. They live in their own "variation_values" table

![captura de pantalla de 2013-08-05 13 27 20](https://f.cloud.github.com/assets/452823/909509/0e1d3348-fdc2-11e2-9d79-35bf2a81c112.png)