add new module purchase_transport_document

This module adds a new object transport.document and allows to choose
those in the purchase order.

- [x] improve doc
- [x] add demo data
- [x] add security rule
- [x] demo -> data noupdate="1"
- [x] make name translatable
- [x] add POT
Bill of lading not landing
http://en.wikipedia.org/wiki/Bill_of_lading