Adding new/updated classes for handling attachments in the reader.

Ports WPRichTextImageControl to swift as WPRichTextImage
Creates a UIWebView backed WPRichTextEmbed class for handling embedded content.
Adds supprt for loading html strings and setting a fixed hegiht.
Implements improved embed handling in rich text views.
Deletes obsolete classes.
Updates delegates & references. Pass content as NSString.

Cleans up obsolete delegate method implementations and updates references to the correct class.
Passes content as a string to the rich text control, instead of an attributed string. Needed for correct html parsing when searching for attachments.
Fixes a bug getting the range of an attachment's markup.
Remove a return used while debugging.
Fixes for video embeds. Improves videopress support.
Merge branch 'develop' of github.com:wordpress-mobile/WordPress-iOS into issues/2252-better-embeds

Conflicts:
	WordPress/WordPress.xcodeproj/project.pbxproj
Merge triage.
Swift code style tweaks
Fixes issues with cliped content and incorrect 'unloaded' frame.
Tweaks initial default viewport setting.
Merge pull request #2537 from wordpress-mobile/issues/2252-better-embeds

Issues/2252 better embeds