Code Cleanup

- Cmake was not updating HAVE_ST_BIRTHTIME.
- NIBBLER_FEATURE_DATE was not properly applied everywhere.
- FEATURE_COLOR was not properly set.
- Some source files failed to include cmake.h, and therefore were not properly
- Removed inefficient use of std::string::substr for guaranteed single character
  strings.
- Integrated Directory::cd.
- Integrated File::ctime, ::btime.
- Integrated Path::operator+.
- Integrated Nibbler::getDigit{2,4,6}.
- Integrated HighResTimer.
  enabling/disabling code.
- All Path objects now expanded internally to absolute form.
- Modified unit tests to accomodate absolute paths.
- Merged new nibbler.t.cpp tests.
- Made various methods const.
- Includes removed from some files, added to others.

src/Directory.h

@@ -43,17 +43,18 @@ public:
   Directory& operator= (const Directory&);
 
   virtual bool create ();
-  virtual bool remove ();
+  virtual bool remove () const;
 
   std::vector <std::string> list ();
   std::vector <std::string> listRecursive ();
 
   static std::string cwd ();
   bool up ();
+  bool cd () const;
 
 private:
   void list (const std::string&, std::vector <std::string>&, bool);
-  bool remove_directory (const std::string&);
+  bool remove_directory (const std::string&) const;
 };
 
 #endif