Add mentions to commenting

Implements #2208

Implements mentions for commenting in Comment, NotificationDetail, ReaderPosts and ReaderPostDetail pages
Ability to delete mentions with backspace (whole word) for UITextView
Suggestions are retrieved per site and cached in a similar manner with NSCache
Fixes a small issue in ReplyTextView's text property which now returns the internal textView's text

@sendhil may I bother you with a review?

/cc @jleandroperez & @allendav