Implement work order functionality

This is work order's first version:
* The initial domain (WorkOrder, WorkOrderItem, WorkOrderCategory) is
implemented.
* Some editors/searchs for those domains, like
WorkOrderEditor/WorkOrderSearch/WorkOrderCategoryDialog/etc.
* A new application for stoq called "Maintenance" that will aggregate all work
order's functionality inside it. In it it's possible to search for work
orders, edit and finish them.
* It's possible to close finished work orders on POS.

Fixes: http://bugs.async.com.br/show_bug.cgi?id=2126
Change-Id: I69e4b0bb30109eeace769de4e93911a2f525a92f