Use relative dates with tooltips fopr Issue#show and MR#show

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/views/projects/issues/show.html.haml

@@ -2,8 +2,7 @@
   Issue ##{@issue.iid}
 
   %small
-    created at
-    = @issue.created_at.stamp("Aug 21, 2011")
+    created #{time_ago_with_tooltip(@issue.created_at)} ago
 
   - if @issue.closed?
     %span.state-label.state-label-red Closed

app/views/projects/merge_requests/show/_mr_title.html.haml

 %h3.page-title
   = "Merge Request ##{@merge_request.iid}"
   %small
-    created at
-    = @merge_request.created_at.stamp("Aug 21, 2011")
+    created #{time_ago_with_tooltip(@merge_request.created_at)} ago
 
   - if @merge_request.merged?
     %span.state-label.state-label-green