More uses of instancetype for initWithFrame

Update method signature of “initWithFrame” to use instancetype instead
of id.
Fix warning on viewDidUnload, deprecated iOS 6.0.

The static analyser picked up on the missing call to super in the
example view controllers viewDidUnload method.  This has been added.
Also note that viewDidUnload is never called on iOS 6.0 and above
because views are no longer purged under low-memory conditions.