Updated AsyncTestHelper

Allow waiting for multiple events
Update CoreDataTestHelper to new concurrency type
Fixes for Photon URLs including schema
Use disk cache for WPTableImageSource
Cache max size of downloaded images
Added some extra helpers to AsyncTestHelper
Refactor downloading of images

Moved the downloading code from WPTableImageSource to WPImageSource, so it can be reused for avatars
Added WPAvatarSource to fetch avatars
Use WPAvatarSource to fetch avatars for reader
Merge branch 'reader' of github.com:wordpress-mobile/WordPress-iOS into reader

Conflicts:
	WordPress/Classes/ReaderPost.m
	WordPress/Classes/ReaderPostTableViewCell.m