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/Date.cpp

@@ -133,8 +133,10 @@ Date::Date (const std::string& input, const std::string& format /* = "m/d/Y" */)
 
   // Parse a formatted date.
   Nibbler n (input);
+#ifdef NIBBLER_FEATURE_DATE
   if (n.getDate (format, _t) && n.depleted ())
     return;
+#endif
 
   // Parse an ISO date.
   if (n.getDateISO (_t) && n.depleted ())