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/CmdHelp.cpp

@@ -125,6 +125,15 @@ int CmdHelp::execute (std::string& output)
            "or at http://taskwarrior.org"
          + "\n"
          + "\n"
+
+         + "<filter>\n"
+           "  Used to restrict the visible data.\n"
+           "\n"
+
+         + "<modfications>\n"
+           "  Changes to apply to the filtered data.\n"
+           "\n"
+
          + "ID is the numeric identifier displayed by the 'task list' command. "
            "You can specify multiple IDs for task commands, and multiple tasks "
            "will be affected.  To specify multiple IDs make sure you use one "