Nuke 2.1.0

• #57 ImageDecompressor now uses CGImageAlphaInfo.NoneSkipLast for opaque images
• Add ImageProcessorWithClosure that can be used for creating anonymous image filters
• ImageLoader ensures thread safety of image initializers by running decoders on a NSOperationQueue with maxConcurrentOperationCount=1. However, ImageDecoder class is now also made thread safe.