Dexterity integration

This pull-request contains code-rewrite to make p.a.imagecropping work both with Archetypes and dexterity. Some details of the changes:

* The CroppingUtil for dexterity was fixed, so that identifying image fields now works correctly.
* The editor template now uses the @@imaging view to display the scaled versions
* A new method `save_cropped` was added to the CroppingUtil. It gets called by CroppingView._crop and handles the actual scale creation and storage, depending if it's for AT or dexterity
* The check if dexterity is installed now follows the recommended way of plone.api

I ran the tests for 4.1, 4.2 and 4.3, all are still passing.
I also tested manually on a 4.1 and 4.2 site, and Plone 4.3 with plone.app.contenttypes installed (= completely AT free). Image cropping, storing, and dislplaying still works in all those sites.