#Refactoring in MMFlowViewImageFactory

- silenced warning for unused parameter -createCGImageForItem:completionHandler:
#Refactoring in MMFlowViewImageCacheSpec

- renamed variable for better test readability
Added more tests for MMFlowViewImageCaches behavior
Support for removing images from MMFlowViewImageCache
#Refactoring in MMFlowView

- changed image cache from plain NSCache into MMFlowViewImageCache
#Refactorings in image decoders

- added tests for setting maxPixelSize
- added tests for creating images with zero maxPixelSize
Removed unneeded comments from private MMFlowView header
Better maxImageSize in MMFlowViewImageFactory

- maxImageSize only accepts positive values
- tests
Image cache shared in MMFlowView and its image factory
#Refactoring

- better test for updateImages