[IMP+FIX] Complete cleaning of point_of_sale module

   - refactorisation of the code: 2000 lines of code removed
   - fixed all the bugs
      - support for tax included/excluded prices
      - fixed rounding troubles
      - don't print the ticket if partial payment
   - cleaning features:
      - workflow reviewed: New -> Done -> Posted / Invoiced
      - generation of journal entries/invoice is now correct
      - only one picking per PoS order
      - wizard to apply global discount (on each lines)
   - useability:
      - reviewed the main point_of_sale screen
      - statements are open by default
   - removed non-useful features
      - advances (you use a special product for this)
      - contracts numbers
      - refund with exchange (you just create a new sale with negatives qty)
      - removed validation steps
      - scan product wizard
   - clean and complete YAML tests (work in progress)

bzr revid: fp@tinyerp.com-20110925140930-s6zibvn51k3914in