build base models custom methods: Common, Invoice, Recurring and Estimate

[JoeZ99]

Give to the following entities:

• Invoice
• Recurring
• Common

The custom methods they have in previous siwapp.

Also make the tests.
@Gissues:{"order":98.80952380952381,"status":"backlog"}

[JoeZ99]

So far, the model is like this

CoreBundle:
- AbstractInvoice
- AbstractItem
- Tax

AbstracInvoice and AbstractItem are both supermap class, they are not Entities. Tax s a regular entity.

Then , for the InvoiceBundle , there is an "Invoice" which inherits from 'AbstractInvoice", an "Item" which inherits from "AbstractItem" . "Invoice" is one_to_many to "Item", and "Item" is unidrectional many_to_many to "Tax" through the specific table "InvoiceItems_Taxes"

The same applies for "RecurringInvoiceBundle" and "EstimateBundle" , each one with specific relationship tables.