added OrderItemsMixin with #addOrderItem to avoid duplicate items on venta's orderItems
using new `addOrderItem` method
added Bandango.NumberField
forcing OrderItem cantidad to be a number
styles
added accounting

see http://josscrowcroft.github.com/accounting.js/
added OrderItemBaseMixin
added numberToCurrencyHelper
styles to tables
replacing number for input
matching styles
Bandango.NumberField should be of type `number`