Added method ImageWorkshopLayer::resizeToFix()
This methods resizes an image to fit a bounding box.
Added method ImageWorkshopLayer::cropToAspectRatio()
This methods crop either to width or height of the document to match the aspect ratio.
Fixed positioning in cropToAspectRatio.
50% should mean the middle of the picture.
Merge pull request #37 from jasny/additional_resizing

Added resizeToFit and cropToAspectRatio methods