Scroll the current view to top when current tab is selected again.

Determines if the current tab is first showing it's root/top view controller (if it's a navigation controller) and if it is and it's showing a UITableView, then scroll to the top.  Prevents the tab bar controller from having yet again another set of hard-coded indices.
Deleted unimplemented delegate method for the tab bar
Merge pull request #1576 from wordpress-mobile/issue/1103-tabbar-scroll-to-top

Scroll the current view to top when current tab is selected again.