CommentsAsync class added for comments retrieval

-Comments retrieval code ported to use AsyncHttpClient.
-ExtendedComment class supports creation of abstracted or faulted
Comment objects that haven’t been fully loaded yet (‘more’ type links).
-First version of ‘load more’ functionality of comments complete
—Ordering of the newly loaded comments isn’t quite robust enough yet.
Needs more work to correctly handle newly loaded ‘more’ links as well.
-Touch screen mode for sliding menu has been changed to margin