pos_cash module added

The whole logic on the POS terminals should be handled by this new module. It's based on a simple data structure to keep things simple.
In the configuration form can be set up the sequence used for enumerating the receipts, the company (used for the company data on the receipt like address etc.), the logo which will be printed and the printer port the receipt printer is connected to.

All operations like adding  or removing products will be added into pos_cash.sale.lines.
Every pos_cash.sale.line appears on the receipt with a tax code. At the end of the receipt a table of taxes is printed. With this table customers can translate the tax codes from the sale.lines to the real tax.
The receipt_code + company.id is printed at the end of the receipt as a code128 type B bar code.

After 50 meters of receipt testing, the receipts look quite nice ;)

The module is available at:
http://hg.holtzberg.de/pos_cash

The next things will be:

• Adding reversal functionality 
• Adding workflow to pos_cash.sale
• Adding customer display support
• Writing a kiosk client