Added option to cache FBProfile images beyond their lifetime

This is very helpful when using the class in a UITableView, where
images are replaced when the UITableView scrolls. Instead of the cached
image having a ‘weak’ retention, if the class is initialised by
correctly, the cached images will have a ‘strong’ retention. Every 30
seconds, the cache will be reduced to the maximum number of images that
was passed on initialisation.
The number of max images to cache beyond the lifetime is now set on a class level instead of a per-instance basis. The cleaning cache mechanism also makes sure the items that were not used recently are deleted first.