sale_validity for v8
remove active in __openerp__.py
reactivate sale_cancel_reason
port models and method to api 8.0
port models and method to api 8.0
reactivate sale_exceptions modules
add tests
port models and methods to api 8.0
Merge pull request #28 from yvaucher/8.0-sale_validity

8.0 port sale_validity
Merge pull request #29 from yvaucher/8.0-sale_cancel_reason

8.0 port sale_cancel_reason
Merge pull request #32 from yvaucher/8.0-sale_exceptions

8.0 port sale_exceptions
fix error with >1 draft SO are checked

when the wizard would run, then the @api.multi method `detect_exceptions` would
fail because it returns an attribute but it is called on a recordset of size >1

Code refactoring to:

* move the side effect of writing to so.exception_ids one method up
* document the side effect in the docstring
* fix the computation of the return value for the `detect_exceptions` method