Layout system optimizations

Computing size for labels is the most time consuming part of the layout system.
We use some metrics to predict wether or not the size should be computed using the last size computed and the new bounding box so that we can know if computing the size is required or not.
WebRequest : Enable cache for images only until we implement the full caching process
CKTableViewController : Reload the table view in view willAppear on iOS7 instead of calling [self reload] that can execute unusefull code at this step
UILabel : Layout Updates optimizations.