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/CmdModify.cpp

@@ -76,7 +76,7 @@ int CmdModify::execute (std::string&)
     Task before (task);
     task.modify (Task::modReplace);
 
-    if (before != task)
+    if (before.data != task.data)
     {
       // Abort if change introduces inconsistencies.
       checkConsistency(before, task);