Renamed ImageFile to UploadFile and modified iOS actual to use NSData instead of UIImage

[rohitst]

A PR for issue #59

The change is breaking, but I think it's worth it.

• Made putObject take an a generic 'UploadFile' instead of image
• Renamed ImageFile to UploadFile
• Changed UploadFile iOS implementation to be backed with NSData instead of UIImage.
• Added an extension UIImage.toPNGUploadFile() to support the image case on iOS

[estivensh]

approved, I will be making some changes so that in the samples folder we can see the instantaneous changes of the modules and not have to wait for a new version.