initialise kPNGSignatureData data

in SDImageCache initWithNamespace method.
This ensures the kPNGSignatureData is always initialised even if we
alloc SDImageCache without using the sharedImageCache singleton
this should take care of issues [26](https://github.com/rs/SDWebImage/issues/26),[55] (http://github.com/rs/SDWebImage/issues/55),[878] (http://github.com/rs/SDWebImage/issues/878)