[ADD] print, print_docsaway, print_sale : module to print and send docum...

...ents by post, implementation for docsaway webservices.

Print and send documents by post
print : provider base module to print account.invoice
print_sale : print and send sale.order
print_docsaway : implementation of docsAway Provider
Is it normal to set 'sent' to True ? Is this field not used for emails or something like that ? Should we have a print_sent field dedicated to printing ? And/or shouldn't we at least have a print_post_process method containing this write allowing to override and udpate this behavior ?