Added __repr__ to EditorsPicK
Make sure all editors picks are resaved on edit. Fixes #408

As we are setting the sort_order manually. The formset doesn't think the editors pick has changed so doesn't save it.
This caused a SQLite test to fail as NULL is always greater than any integer causing ordering to be incorrect.
Improved test_post_reorder test
Merge branch 'issue/408' of https://github.com/kaedroho/wagtail into kaedroho-issue/408
Merge branch 'kaedroho-issue/408'