preserving orientation problem in scaleByFactor: method

My previous pull request (#19 issue) added a problem in the resizing code. Orientation should not be preserved if orientation handling code (92-107 lines) is executed. My suggestion is just to preserve initial orientation, not to rotate the image.

 + IB like 'aspect fill' and 'scale to fit' methods
They don't take orientation into account, user is responsible for it

+ little refactor
CG work is done only in scaleToFillSize: method, all other methods use scaleToFillSize: