Allow user to select EGOv1 style PNG only caching 

We have a enterprise endpoint which was causing a throw on most invocations of [NSKeyedUnarchiver unarchiveObjectWithFile:  

ARC leaks on Exception throws so each of the many throws was eventually shutting down App

Dont know why inbound data (a mix of PNGs and JPG) wasn't archiving correctly and cant change the endpoint data so I just restored the original EGOv1 UIImagePNGRepresentation(anImage) storage for this.

- (UIImage*)imageForKey:(NSString*)key
- (void)setImage:(UIImage*)anImage forKey:(NSString*)key  
- (void)setImage:(UIImage*)anImage forKey:(NSString*)key withTimeoutInterval:(NSTimeInterval)timeoutInterval