fix number localization

`Numeric.parse_localized` returns a String that cause a wrong conversion when mysql store the field. i.e.

Without casting to float
```
Started POST "/payments"
Parameters: {"utf8"=>"✓", "authenticity_token"=>"/xzRBkw+5eqiuW9tvKYNV8pGhUztmt73WNTcYw8gzI8=", "payment"=>{"invoice_id"=>"564", "owner_id"=>"2103", "owner_type"=>"DemoRequest", "amount"=>"3.696,30", "paid_at(3i)"=>"4", "paid_at(2i)"=>"10", "paid_at(1i)"=>"2013", "payment_type_id"=>"1", "bank"=>"Barclays", "ref"=>"asdasd"}, "commit"=>"Salva"}

INSERT INTO `payments` (`amount`, `bank`, `created_at`, `invoice_id`, `owner_id`, `owner_type`, `paid_at`, `payment_type_id`, `ref`, `updated_at`) VALUES (369630.0, 'Barclays', '2013-10-04 15:00:02', 564, 2103, 'DemoRequest', '2013-10-04', 1, 'asdasd', '2013-10-04 15:00:02')
```
Note that "amount"=>"3.696,30" is stored as 369630.0 instead of 3696.30