Prevent unnecessary writes on sales orders

When the exceptions are the same, do not write them again,
this prevent to write on sales orders when it is not necessary.
Column name is exceptions_ids, not exception_id
[IMP] add multi-company support in module sale_exceptions
[FIX] faulty default function signature
sale_validity: backport 8.0 tests to 7.0
Merge pull request #31 from nbessi/sale_ex_multi_comp

[IMP] add multi-company support in module sale_exceptions
Merge pull request #17 from guewen/sale-exception-useless-write

[FIX] sale_exceptions - Prevent unnecessary writes on sales orders
Merge pull request #36 from gurneyalex/7.0

sale_validity: backport 8.0 tests to 7.0
remove call to travis_run_flake8 from .travis.yml
fix mutable object as method arg default value

use states=None in sale_order.action_invoice_create() which is interpreted in
the parent class implementation as meaning ['confirmed', 'done', 'exception']
Merge pull request #45 from gurneyalex/7.0-fix-pylint

fix mutable object as method arg default value
Add module last_sale_price
improve module description
remove __init__.py in test
English correction typo and improvement of description
quantity instead of qty
this module is more a feature than an application
be zero division safe
extract all modules depending on sale_stock to test them separatly
Merge pull request #14 from yvaucher/port-7.0-add-last_sale_price-yvr

[ADD] last_sale_price