Expressions

- Implemented Nibbler::getWord.
- Re-implemented Nibbler::getDOM.
- Modified DOM addressed for context-based attributes from "due" to ".due",
  the help disambiguate DOM references in expressions.  There is now a
  consistency:

      <id>.due         Task-specific
          .due         Contextual
    <uuid>.due         General

- Implemented associated unit tests.

src/DOM.h

@@ -43,7 +43,7 @@ public:
   void set (const std::string&, const std::string&);
 
 private:
-  bool is_primitive (const std::string&);
+  bool is_literal (std::string&);
 
 private:
   std::map <std::string, std::string> _cache;