Lexer

- Implemented Lexer::word, which is just like ::token, but does not
  understand dates, durations or operators.
- Implemented Lexer::split, which uses Lexer::word.
- Added unit tests.

src/Lexer.h

@@ -58,10 +58,12 @@ public:
   Lexer& operator= (const Lexer&); // Not implemented.
   bool operator== (const Lexer&);  // Not implemented.
   bool token (std::string&, Type&);
+  bool word (std::string&, Type&);
   void ambiguity (bool);
 
   static const std::string type_name (const Type&);
   static bool is_ws (int);
+  static void split (std::vector <std::string>&, const std::string&);
 
 private:
   bool is_punct (int) const;