Make the configlet setting for quality take effect.

Add getQuality for registration of utility
(providing IScaledImageQuality from plone.scale)
Test the quality setting in an integration test.
Test that lowering quality makes an image smaller.
Added documentation on re-scaling existing scales.
Patch ImageField's scale method.

We want to use our site's scaling quality setting
in there.
Without this patch, the setting has no effect in
a standard Plone site.