First pass at following site via URL

Groups site related methods into its own service.
Stubs a FollowedSitesViewController.
Adds check for modal view controller.
Adds ReaderSite model and fleshes out followed sites vc
Wires up followed sites view controller.

Shows sites the user is following.
Supports unfollowing sites from the list.
Adds a no results view.
Merge branch 'develop' of github.com:wordpress-mobile/WordPress-iOS into issues/1976-follow-blog-by-url

Conflicts:
	WordPress/WordPress.xcodeproj/project.pbxproj
Workaround for apparent REST API bug. Adds safety net.
Adds images for feed and followed toast.
Save context when merging sites.
If site has no name use its url.
Adds checks for existing url, and if already subscribed.
Fix substring starting index.
Corrects some layout issues when first displaying cells.

Displays a consistently sized blavatar.
Moves property out of public header. Removes commented lines.
Simplify if conditional.
Fix method parameter description.
Fix method name case.
Clarify variable names.
Simplify if conditionals. Fix comment typo.
Update enum declaration.