Better image preview processor

 * Thumbnail and wide images are converted to JPG (transparent pixels are set to white)
 * GIF images are converted to JPG
 * Other images retain their original format
Fixed bug in storing principal profile picture. We use  rather than  for multipart file objects.
Made convertToJPG an ImageUtil utility.
It is now being used for both content preview thumbnails as profile pictures.

When the user uploads his profilepicture, it will be converted to a plain JPG.
Add documentation for image uploading.
#675 - Improved JSDoc
Merge pull request #675 from simong/675-pp-images

Preview Processor Issue with PNG/GIF images that contain transparent pixels