Total rework with CoreData

New logic using Core Data (inspired by
https://github.com/vascoorey/iOS-Projects/tree/master/Photomania).
Improve image download.
Merge branch 'alternative-logic'

Conflicts:

DARPFlickrAroundYou/DARPFlickrAroundYou.xcworkspace/xcshareddata/DARPFli
ckrAroundYou.xccheckout

DARPFlickrAroundYou/DARPFlickrAroundYou/Class/Logic/DARPPhotosDownloadMa
nager.m