Fix incorrect handling of userstory rolepoints update process.

The current implementation allows duplicate points values in one project,
and because of it, unexpected MultipleObjectsReturned can be raised with
previous update_role_points code. The new one fixes this.

Additionally, in some circumstances, the project does not have any points
with None as value, that causes also unexpected errors. The new
implementation fixes it creating a points instance with None as value
if it not exists.
Merge pull request #126 from taigaio/rolepoints-update-fixes

Fix incorrect handling of userstory rolepoints update process.