Android 4+ bug : use of JpegEncoder

.toDataUrl always return png on Android, and file.type="". To convert
the resized canvas to jpeg, we use JpegEncoder.

More info on :
http://ghinda.net/jpeg-blob-ajax-android/

With my other pull request pending on DarrenInWood/Jquery-File-Upload (still to migrate in separate files), it is now possible to resize and post images on Android 4+