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/A3.cpp

@@ -1575,6 +1575,7 @@ bool A3::is_dom (Nibbler& n, Arg& arg)
 ////////////////////////////////////////////////////////////////////////////////
 bool A3::is_date (Nibbler& n, std::string& result)
 {
+#ifdef NIBBLER_FEATURE_DATE
   std::string date_format = context.config.get ("dateformat");
   std::string::size_type start = n.save ();
   time_t t;
@@ -1586,6 +1587,7 @@ bool A3::is_date (Nibbler& n, std::string& result)
   }
 
   n.restore ();
+#endif
   return false;
 }