Add a keepAspectRatio optional parameter to GetImage methods

Changing how cropping was done to fix the memory issues from cropping a small signature changed how cropping worked on Android.  Previously on Android, when an image was cropped, it removed all extra space, thus changing the aspect ratio of the resulting image from that of the original control.  On iOS and WP, the aspect ratio was maintained, so there could still be some empty space on the right or bottom of the cropped signature.  As part of the change to fix the memory issue, Android now does the same by default.

The keepAspectRatio allows the developer to decide which way they want it to behave, so they can choose to keep the aspect ratio or remove all empty space as they desire on any of the three platforms.