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.cpp

@@ -91,13 +91,13 @@ bool Directory::create ()
 }
 
 ////////////////////////////////////////////////////////////////////////////////
-bool Directory::remove ()
+bool Directory::remove () const
 {
   return remove_directory (_data);
 }
 
 ////////////////////////////////////////////////////////////////////////////////
-bool Directory::remove_directory (const std::string& dir)
+bool Directory::remove_directory (const std::string& dir) const
 {
   DIR* dp = opendir (dir.c_str ());
   if (dp != NULL)
@@ -191,6 +191,12 @@ bool Directory::up ()
   return false;
 }
 
+////////////////////////////////////////////////////////////////////////////////
+bool Directory::cd () const
+{
+  return chdir (_data.c_str ()) == 0 ? true : false;
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 void Directory::list (
   const std::string& base,