Fix Issue Where Large Images Get Vertically Squashed In iOS6 & iOS7

This pull request implements the ```detectVerticalSquash()``` function from the ios-imagefile-megapixel library (https://github.com/stomita/ios-imagefile-megapixel) to fix a bug in iOS6 & iOS7 where large images get vertically squashed when added to the canvas. This issue is likely to occur when adding image from the iPhone file system to the canvas.

Function is called on the image only if the ```Kinetic.UA.browser``` indicates that the browser is webkit, and is not called on cross origin images or images with an SVG source as ```getImageData()``` would raise a security exception.

A test was also added to confirm that the image is not being vertically squashed.