Added api to assign discount codes for associated

We check e-mail against associated site and assign
that e-mail to one valid promo code in the database

Signed-off-by: Fernando Rocha <fernandogrd@gmail.com>
Added e-mail logic of discount code

Signed-off-by: Fernando Rocha <fernandogrd@gmail.com>
Added pt_BR translations to promo_code app

Signed-off-by: Fernando Rocha <fernandogrd@gmail.com>
Added lock to avoid race conditions

Signed-off-by: Fernando Rocha <fernandogrd@gmail.com>
Merge pull request #15 from fernandogrd/promo_code

Promo code check API