credit users with a bottom limit on order completion date

credit_loyalty_points_to_user accepts a date, so that no orders before
that date will be credited.

Change-Id: Ifec85ac9f49e8b43da66b9caa63e6dd5f558821d
Merge pull request #9 from Dkendal/2-1-stable

credit users with a bottom limit on order completion date