Add allowsCropping property
Add crop preview functionality

* CZPhotoPickerController now accepts a CGSize to set the desired
size of the crop overlay view. Images are centered beneath the crop
overlay view to give you a sense of how the image will be cropped
when it is displayed in an image view of the same size. The edited
image that is returned is cropped such that the aspect ratio matches
the aspect ratio of the provided size.
Pass allowsEditing through to the photo picker
Visual tweaks

* Lighten the dark overlay
* Add a highlight border to the crop region
* Change prompt text to Crop Preview
Fix cropping alignment
Update example
Merge pull request #2 from carezone/ma-preview-scale-crop

Add crop preview functionality