Scaling overrides

- updated buildout to Plone 4.3
- adding custom scaling views for image-type, news-type and plone.namedfile
- changed api of save_cropped method to remove redundant parameter

supersedes #37, fixes #21 and probably even fixes #24

at a first glance this seems to work. @frisi, @petschki, @hvelarde can someone review this please?