Task: Remove std::map inheritance, clean up interface

- Make the Task object's interface more explicit by removing the
  std::map inheritance.
- Using this more explicit interface, remove unneeded ctors in order to
  allow the compiler to "Do The Right Thing"(tm).
  This leads to a performance improvement of 12% in the "add"
  performance test, and 7% for "import".

src/commands/CmdImport.cpp

@@ -205,7 +205,7 @@ void CmdImport::importSingleTask (json::object* obj)
     if (hasGeneratedEnd)
       task.set ("end", before.get ("end"));
 
-    if (before != task)
+    if (before.data != task.data)
     {
       CmdModify modHelper;
       modHelper.checkConsistency (before, task);