Task Refactoring

- Task is no longer a map of string to Att.  Att is itself a name/
  value pair, so the name was redundant.  Task is now a map of string
  to string.  This brings the obsoletion of Att much closer.

src/commands/CmdStatistics.cpp

@@ -133,7 +133,7 @@ int CmdStatistics::execute (std::string& output)
 
     descLength += task->get ("description").length ();
 
-    std::vector <Att> annotations;
+    std::map <std::string, std::string> annotations;
     task->getAnnotations (annotations);
     annotationsT += annotations.size ();