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.

test/att.t.cpp

@@ -84,11 +84,7 @@ int main (int argc, char** argv)
   a5.value ("\t\",[]:");
   t.is (a5.composeF4 (), "name:\"&tab;&dquot;,&open;&close;:\"", "Att::composeF4 fully encoded \\t\",[]:");
 
-  Att a6 ("name", 6);
-  t.is (a6.value_int (), 6, "Att::value_int get");
-  a6.value_int (7);
-  t.is (a6.value_int (), 7, "Att::value_int set/get");
-  t.is (a6.value (), "7", "Att::value 7");
+  Att a6 ("name", 7);
 
   // Att::mod - straight comparisons.
   bool good = true;