TDB2

- Implemented CmdAdd.cpp and CmdLog.cpp using TDB2.
- Implemented simple append writes in TDB2.
- Modified CmdImport to accept and parse JSON.
- Added more const-ness in DOM, Expression and Task, to allow TDB2::get_tasks
  to return a const vector ref, which is a Very Good Thing.
- Corrected usage for the export command.
- Implemented Task::urgency as a call to Task::urgency_c, which is a const
  overload to allow urgency calculations (without caching) for const Task
  objects.
- Removed obolete code from TDB.
- Added lots of diagnostic output for TDB2 - it's annoying, but will be
  gone soon.
- Added mention in CmdHelp of the new <filter> and <modifications>
  syntax elements.  Needs more.
- Added Command::filter overload which uses TDB2.  Not in use yet.

src/Expression.cpp

@@ -66,7 +66,7 @@ Expression::~Expression ()
 }
 
 ////////////////////////////////////////////////////////////////////////////////
-bool Expression::eval (Task& task)
+bool Expression::eval (const Task& task)
 {
   // If there are no elements in the filter, then all tasks pass.
   if (_args.size () == 0)