Change the way chunks are moved between regions.
Less efficient in terms of database updates, but more reliable because
I suck at maths.

Squashes 4dd84434c0af24def7437e3a9cfd4f72f6a21d46 and a15c38518a7437235de5ff45001dff4850d00abd.

Original branch may still be at: https://github.com/kezabelle/django-editregions/tree/tickets/2

Fixes #2.