Make ImageCropField widget overwritable

It's important so we can create custom widgets to ImageCropField
Merge pull request #27 from sayadn/master

Make ImageCropField widget attribute overwritable
Use ImageCroppingMixin in tests
fix help_text if fields are on the same row
Merge remote-tracking branch 'judw/master' into free_cropping
Merge branch 'free_cropping' into artur
Merge branch 'fix_help_text' into artur