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/commands/CmdImport.h

@@ -31,6 +31,14 @@
 #include <string>
 #include <Command.h>
 
+class CmdImport : public Command
+{
+public:
+  CmdImport ();
+  int execute (std::string&);
+};
+
+/*
 class CmdImport : public Command
 {
 public:
@@ -62,6 +70,7 @@ private:
   std::string CSV (const std::vector <std::string>&);
   std::string YAML (const std::vector <std::string>&);
 };
+*/
 
 #endif
 ////////////////////////////////////////////////////////////////////////////////