Huge refactoring in image loading

- cleaner MMImageDecoderProtocol for better encapsulation
- refactored all image decoders for adapting the image decoder protocol
- removed cache from MMFlowViewImageFactory, enforcing SRP
- cleaner tests for all image loading related code
- updated cocoa pods
Fixed memory leaks in image decoders
Added missing MIT license header to some source files

- removed unused string contants
Reordered MMFlowView.h