Fetch data in disk cache
Update access date when fetching data from disk cache
setImage -> fetchImageForKey
Fix isEqualPixelByPixel
Fetch image from disk cache if needed
Use implicitly unwrapped optional like in UIImagePNGRepresentation
Add missing tests
Haneke struct for static constants
Object not found error
Merge pull request #43 from Haneke/feature/disk-cache/fetch

Fetch images from disk cache. Closes #6. Closes #13.