UI for viewing photos.

Added functions:
Display all photos’ thumbnails in collection view.
Click thumbnail to view full resolution  photo.
Move and zoom photo.
Swipe to switch between photos.
Single tap to switch to and back from full screen mode.
Double tap to zoom in or out.

Remained bug:
Double tap will also trigger single tap.
Zooming size is not exact.

Code clean ups:
All except WSAlbumsTVC.h, WSAlbumsTVC.m, WSAlbumPVC.h and WSAlbumPVC.m.

All strings that will displayed on UI to user and strings that are used as identifier in storyboard are move to DisplayString.h and UIIdentifierString.h.

Magic strings that are used to access Core Data should also be redefined, which is not done yet.