Fixed bug where NSInvalidArgumentException was thrown if an image with a nil URL was downloaded.
Removed creation of NSError when a nil URL is downloaded.
Merge pull request #274 from reidmain/master

NSInvalidArgumentException is thrown if a nil NSURL is downloaded.
Fixed CGBitmapContextCreate warnings for invalid alpha properties
Invalid cost calculation when saving image to memory cache
Merge pull request #278 from Reflejo/invalidcost

Invalid cost calculation when saving image to memory cache
Merge pull request #277 from Reflejo/upstream

Fixed CGBitmapContextCreate warnings for invalid alpha properties