Testfix
Load cached gravatar via NSData. Closes #347

The problem was a race condition that occured while loading the cached gravatar while at the same time the avatar cache got cleared. UIImage. imageWithData loads the image as a valid image (instead of returning nil) in case it can read the header. The image data was corrupted when the image got deleted while loading it, that's why only a black gravatar was shown.

For more details, see this answer on SO:
http://stackoverflow.com/a/10685962/183537
Moved avatar cache clearing to didFinishLaunchingWithOptions

The changes do not take immediate effect when doing this in applicationDidBecomeActive, because already cached gravatars on existing user object instances are not purged. This makes it more intuitive.