Changed Facebook Graph query for profile picture.

It is best to ask for a profile picture of a specific size and let Facebook crop the image. This way the picture does not appear stretched. 

There may be other (better?) solutions though:
1. Change the UIImageView mode to Aspect Fit
2. The method `loadUsernameAndProfilePictureOfCurrentUserWithCompleteBlock:andErrorBlock:` should probably accept the needed size of the profile picture as a parameter.