Django-virtual-pos

- Include migrations.

- Allow reverse relation VPOSPaymentOperation -> VirtualPointOfSale

- Adding environment to VPOSPaymentOperation
- Changing labels in models

- Fixing get_type_help_bug

- Add new permission view_virtualpointofsale to ease management.
- Add method specificit_vpos in VirtualPointOfSale that returns the specific model object according to the VPOS type.

Minor changes in README.md.