Enforced presence of a completedBlock in downloadWithURL:options:progress:completed

This method bails out in case of a missing `completedBlock`. While this makes sense (downloading the image and not doing anything with it is pointless), a client passing a `nil` block is not informed about the mistake.

`NSParameterAssert` validates the input and informs the client about bad usage of the API.
Then the NSAssert should print some kind of message like 'Please use the prefetcher if you just want to download the image...'