add new module purchase_transport_document

This module adds a new object transport.document and allows to choose
those in the purchase order.
add security, doc, demo data
demo -> data

Those examples of transport documents are fairly common, so I always
include them (not only as demo data).
fix typo in manifest
make name translatable, add i18n template
lading is a word!
move doc to README.rst
Merge pull request #35 from lepistone/purchase_transport_document

add new module purchase_transport_document